CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL RECORD_TYPE = STREAM PRODUCT_CREATION_TIME = 1991-09-23 OBJECT = TEXT NOTE = "Introduction to the Mars Mosaicked Digital Image Model (MDIM) CD-ROM volumes." END_OBJECT = TEXT END Mars Mosaicked Digital Image Model (MDIM) and Digital Terrain Model (DTM) Eric Eliason, Raymond Batson, Anthony Manley Branch of Astrogeology United States Geological Survey 2255 North Gemini Drive Flagstaff, Arizona 86001 August 1, 1991 Version 1.0 CONTENTS 1 - INTRODUCTION 2 - VIKING MISSION 3 - VIKING ORBITER VISUAL IMAGING SUBSYSTEM 4 - CARTOGRAPHY AND DATA PRODUCTS 5 - DATA PREPARATION: PLANETARY DIGITAL MODELS 5.1 - PROJECTIONS 5.2 - PIXEL SIZES 6 - COMPILATION OF DIM'S 6.1 - LEVEL 1: RADIOMERIC CORRECTION 6.2 - LEVEL 2: GEOMETRIC CORRECTION 6.3 - LEVEL 3: PHOTOMETRIC CORRECTION 6.4 - LEVEL 4: CONTROLLED MOSAICKING 6.5 - DENSITY CONTRAST OF MDIM IMAGES 6.6 - MDIM IMAGE "ARTIFACTS" 7 - CONCEPT OF TILING SCHEME 8 - FILES, DIRECTORIES, AND DISK CONTENTS 8.1 - IMAGE FILE NAMING CONVENTION 8.2 - DIRECTORIES 9 - IMAGE FILE ORGANIZATION 9.1 - IMAGE LABEL AREA 9.2 - IMAGE HISTOGRAM OBJECT 9.3 - IMAGE OBJECT 10 - SOFTWARE 10.1- SOFTWARE DISCLAIMER 10.2- SOFTWARE TOOLS 11 - IMAGE INDEX 12 - GAZETTEER 13 - ACKNOWLEDGEMENTS 14 - REFERENCES APPENDIX A - ISO VOLUME AND DIRECTORY STANDARD APPENDIX B - SYNTACTIC RULES OF KEYWORD ASSIGNMENT STATEMENTS APPENDIX C - KEYWORD ASSIGNMENTS FOR MDIM IMAGES APPENDIX D - GEOMETRIC DEFINITION OF A PIXEL APPENDIX E - SINUSODIAL EQUAL-AREA PROJECTION EQUATION 1 - INTRODUCTION This digital image map of Mars is a cartographic extension of a previously released set of CDROM volumes containing individual Viking Orbiter Images (PDS volumes VO_1001, VO_1002, etc.). The data in the latter are pristine, in the sense that they were processed only to the extent required to view them as images. They contain the artifacts and the radiometric, geometric, and photometric characteristics of the raw data transmitted by the spacecraft. This new volume set, on the other hand, contains cartographic compilations made by processing the raw images to reduce radiometric and geometric distortions and to form geodetically controlled Mosaicked Digital Image Models (MDIMs). (Because the photometric processing used in this MDIM was oversimplified, quantitative radiometric analysis on this data is not possible.) It also contains digitized versions of an airbrushed map of Mars as well as a listing of all IAU-approved feature names. In addition, special geodetic and photogrammetric processing has been performed to derive rasters of topographic data, or Digital Terrain Models (DTMs). The latter has a format similar to that of the MDIM, except that elevation values are used in the array instead of image brightness values. The MDIM CDROM collection serves two purposes. First, the image collection serves as a data base for interactive map browser applications. Secondly, the CDROM volume set provides a dense delivery medium to build higher-derived cartographic image products such as special map series and planning charts for the Mars Observer Project. This set contains seven volumes with the following contents: Volume 1. Vastitas Borealis Region of Mars (VO_2001): MDIMs in 373 image files covering the entire north polar region of Mars southward from the pole to a latitude of 42.5 deg. North. Polar Stereographic projection images of the north pole area from 80 to 90 degrees are located in the POLAR directory on this disk Volume 2. Xanthe Terra Region of Mars (VO_2002): MDIMs in 412 image files covering the region of Mars from 47.5 deg. North latitude to 47.5 deg. South latitude, and 0 deg. longitude to 90 deg. West longitude. Volume 3. Amazonis Planitia Region of Mars (VO_2003): MDIMs in 412 image files covering the region of Mars from 47.5 deg. North latitude to 47.5 deg. South latitude, and 90 deg. West longitude to 180 deg. West longitude. Volume 4. Elysium Planitia Region of Mars (VO_2004): MDIMs in 412 image files covering the region of Mars from 47.5 deg. North latitude to 47.5 deg. South latitude, and 180 deg. West longitude to 270 deg. West longitude. Volume 5. Arabia Terra Region of Mars (VO_2005): MDIMs in 412 image files covering the region of Mars from 47.5 deg. North latitude to 47.5 deg. South latitude, and 270 deg. West longitude to 0 deg. West longitude. Volume 6. Planum Australe Region of Mars (VO_2006): MDIMs in 373 image files covering the entire South polar region of Mars northward from the pole to a latitude of 42.5 South latitude. Polar Stereographic projection images of the south pole area from 80 to 90 degrees are located in the POLAR directory on this disk Volume 7. Digital Topographic Map of Mars (VO_2007): MDIMs of the entire planet at 1/64, 1/16, DTMs of the entire planet at 1/64, 1/16, and the digitized airbrush map of Mars at 1/16 and 1/4 deg./pixel. Each of the first six volumes contains MDIMs of the areas specified at resolutions of 1/256 deg./pixel (231m) and at 1/64 deg./pixel (943m). Volumes 1 and 6 also contain MDIM coverage of the entire planet at 1/16 deg./pixel (3.69 km). The six volumes also include a digitized airbrush map of the entire planet at 1/16 deg./pixel (3.69 km) and at 1/4 deg./pixel. The Sinusoidal Equal-Area Projection, is used as the map projection for this image collection. For a detailed description of the Sinusoidal projection and use of the cartographic keywords found in the image labels, refer to Appendix E of this document. The tiling layout of the 1/64 deg./pixel digital models is the same on the first six volumes. Note that the 1/64 deg./pixel MDIM, segments of which appear in Volumes 1 through 6, is duplicated in its entirety on Volume 7. All of the resolution compressions were done by averaging, not by subsampling. A gazetteer of IAU-approved feature names, referenced by latitude/longitude coordinates is included as a table file on each of the seven volumes. 2 - VIKING MISSION The Viking Mission consisted of four spacecraft: two identical orbiters and two identical landers. During cruise from Earth to Mars the landers were attached to the orbiters. Thirteen science teams had experiments on these spacecraft. The major scientific objective of the mission was to search for life on Mars. Several experiments on the landers were designed to address this objective. In addition, some of the experiments on the orbiters and landers focused on the study of the composition and physical properties of the atmosphere, the distribution of water vapor, and global and local meteorology. Other experiments investigated the composition and physical properties of the surface and the geologic history of Mars. Data on the seismicity of Mars and its gravity field were also acquired to study the internal structure of Mars [1, 2]. One of the Orbiter experiments was the Visual Imaging Subsystem (VIS), which acquired the images that comprise the Mars MDIM. The imaging system is briefly described in the next section. The first objective of the VIS experiment was to characterize potential landing sites in support of site selection. Additional objectives were to study the photometric and colorimetric properties of the surface; to study various geological features that were discovered by Mariner 9 in order to better understand the geological history of Mars; to study the dynamics of the atmosphere; and to monitor the surface for changes. The Viking Orbiter spacecraft operated in orbit around Mars from 1976 until 1980. The overall Viking mission was divided into a number of mission phases with specific objectives. The time from orbital insertion in June 1974 until November 1976 is known as the Primary Mission. The main objective of the Orbiter instruments was to collect data in support of landing site selection. The spacecraft orbital characteristics were chosen so that the Orbiters could serve as relay stations for communications between the Landers and Earth. In addition, the Orbiter imaging systems imaged all of the terrains on Mars, collected some color and stereo images, and made observations of Phobos and Deimos. The Viking Extended Mission took place from November 1976 through May 1978, and the Viking Continuation Mission took place from May 1978 through February 1979. During these periods the Orbiters were not always required as relay stations with the Landers. Some of the image sequences acquired by the VIS experiment include systematic medium and high resolution coverage of large portions of the surface, stereo images, observations of Phobos and Deimos, color images of the equatorial regions, observations of the polar regions, and monitoring dust storm activity. The final phase of the Viking Mission was the Survey Mission from July 1979 until July 1980. During the Survey Mission only Viking Orbiter 1 operated since Viking Orbiter 2 had lost its attitude control gas through a series of leaks. The Orbiter 1 image coverage during the Survey Mission was designed to obtain contiguous high resolution coverage of the Martian cratered terrain. One reason for acquiring these data was to help select landing sites on Mars for future missions. 3 - VIKING ORBITER VISUAL IMAGING SUBSYSTEM Each Viking Orbiter was equipped with two identical vidicon cameras, called the Visual Imaging Subsystem (VIS) [3, 4, 5]. Each VIS camera consisted of a telescope, a slow scan vidicon, a filter wheel, and associated electronics. The angular field of view of the camera as defined by the reseau pattern was 1.51 by 1.69 degrees. The ground area covered by an image varies as a function of spacecraft altitude and emission angle. A digital image was generated by scanning the vidicon face plate. The signal at each location (pixel) was digitized as a 7-bit number (i.e., within the range of 0 to 127). The EDR image data were converted to 8-bit numbers by multiplying the original 7-bit numbers by 2. Thus, the least significant bit of each pixel in an EDR image is zero, except for interpolated pixels or pixels with corrupted values. A full-resolution, Viking Orbiter image consists of an array of 1056 lines with 1204 samples per line. There are only 1182 valid samples in each line. The extra 22 samples in each line consist of dark bands on the left and right edges of each image, produced by an opaque mask located at the front of the vidicon. Each dark band is approximately 11 samples wide, although the exact width varies from image to image. Each VIS camera contained a filter wheel with five color filters (blue, minus blue, violet, green, and red) and a clear position, i.e., no filter. The filter half power bandwidths are approximately: blue from 0.35 to 0.53 micrometers; minus-blue from 0.48 to 0.70 micrometers; violet from 0.35 to 0.47 micrometers; clear from 0.35 to 0.70 micrometers; green from 0.50 to 0.60 micrometers; and red from 0.55 to 0.70 micrometers. Multiple images of the same areas were occasionally acquired using violet, green, and red filters to form color images after processing on Earth. Color image reconstruction from Viking imaging requires radiometric and geometric corrections, and co-registration of the images that make up the color set. 4 - CARTOGRAPHY AND DATA PRODUCTS The global MDIM of Viking Orbiter images of Mars was compiled according to the plan described by Batson [6, 7, 8, 9]. The images have had improved radiometric and geometric enhancements over the images used in the published 1:2,000,000-scale controlled photomosaic map series published by USGS. The MDIM is tied to a refined topographic control net for Mars [10] with a published standard error of this net of about 5 km for the control base, which represents 20 pixels at 1/256 deg./pixel in the MDIM. Discrepancies between adjacent frames are far less than 20 pixels over most, but not all, of the planet. We attempted to distribute the error so that it was not obtrusive in the mosaics, but this was not possible in some areas. The error can be attributed to a lack of precise knowledge of the spacecraft location at the time each image was taken and to parallax in oblique images of rugged terrain. Camera locations can be derived only by tracking the spacecraft continuously and precisely during its active lifetime, which was not always done for Viking Orbiters 1 and 2. Given assumed camera positions, camera orientations were derived by reducing to their minima the discrepancies between images in overlapping frames and the control net. First-order photometric corrections were also performed and contrast ranges were normalized based on solar illumination geometry. This processing greatly reduces tonal discrepancies between individual images even when illumination differences are extreme, but it is a form of spatial filtration and results in the loss of regional albedo information. 5 - DATA PREPARATION: PLANETARY DIGITAL MODELS Digital mosaics have in the past been compiled primarily as elegant demonstrations of a costly alternative to manual compilations. They have generally been special products designed to serve specialized purposes and until recently were not affordable as primary standard products. The intent of the digital planetary mapping program is to develop a unified system, consisting of a single digital format for all planetary cartographic databases. The relations between digital map-storage formats and map projection and image resolution are therefore fundamental considerations in the design of the system. 5.1 - PROJECTIONS The simplest form of a digital model (DM) is one in which each image element's value is stored in a "bin" (pixel) labeled in terms of latitude and longitude. For computer work, it is only necessary that each bin be readily accessible. In compiling and describing DM's, however, it is useful to discuss a digital array in terms of map projections. The simplest projection is one in which each image line, or row of bins, is a parallel of latitude and each column of samples, or bins, is a meridian. This presentation was termed a "Simple Cylindrical" or "Square" projection by Clark [11]. Its simplicity is appealing, even though the higher latitudes are oversampled (e.g., the pole of a planet, in reality, is a point, but is represented digitally by an image line with as many samples as that for the equator, all with the same value). Several planetary consortia, consisting of geological, geochemical, and geophysical databases in this format, have used this format for several years for the Moon, Mars, Venus, and the Galilean satellites [12, 13, 14, 15]. The total storage required for this kind of array is only about 60% more than if each element represented the same size area on the planet, and is therefore not prohibitive. However, this projection does present an operational problem, in that a Simple Cylindrical projection of a single spacecraft image containing the north or south pole has too many pixels in an image line to manage easily during DM compilation. As a result, the Sinusoidal Equal-Area projection [16] was selected for compiling planetary DM's. The conversions between Simple Cylindrical and Sinusoidal Equal-Area geometry are so computationally trivial that the two formats are nearly twins. A Sinusoidal Equal-Area projection is one in which each parallel of latitude is an image line, and the length of each line is compressed by the cosine of its latitude. For a detailed description of the Sinusoidal projection and use of the cartographic keywords found in the image labels, refer to Appendix E of this document. The Sinusoidal projection has the simplicity of the Simple Cylindrical projection so far as indexing (rows and columns are still parallels along meridians), but compilation is much more efficient in the Sinusoidal Equal-Area because the projection does not have mathematical peculiarities at the poles. However, viewing distortion becomes severe with distance from the central meridian in the sinusoidal presentation. This is a problem only for visual examination of DIM images; it is not relevant to the integrity of the database. By simply sliding image lines parallel to one another, the central meridian can be rapidly shifted; this allows an undistorted view of a selected region without geometrically resampling the image. Segments of a DIM can therefore be displayed with a local central meridian, although the poles themselves must be transformed to a polar projection. 5.2 - PIXEL SIZES The resolution of digital images is often given in terms of pixel dimensions in meters or kilometers on the surface of a target. However, Mars DM's on this CDROM are encoded so that the number of lines (which are also parallels of latitude) in a global DM is an integer. It is therefore more convenient to specify DM resolution in terms of planetocentric degrees than in linear units. The size of pixels in DM's is therefore specified as some negative power of two (1/4, 1/8, 1/16 . . . 1/256, etc.) degrees per pixel. Resolutions intermediate to these values are not used, so that databases can be registered in scale simply by successively doubling or halving the pixel sizes by subsampling or averaging, but without resampling. Selected segments of DIMs may be written as photographic prints or published as maps in the traditional format. Table 1 shows metric equivalents of pixel sizes for each solar system body currently included in NASA's mapping plans [17]. Table 1 shows equivalents of digital model resolutions, in kilometers per pixel. Mean radii are given for non-spherical bodies. TABLE 1. - METRIC EQUIVALENTS OF PIXEL SIZES FOR SOLAR SYSTEM BODIES -------------------------------------------------------------------- | |Radius| Digital scale (deg/pixel) | |Planet | (km) | 1/16| 1/32| 1/64| 1/128| 1/256| 1/512|1/1024| |_________|______|______|______|______|______|______|______|______| | | | | | | | | | | |Mercury | 2439| 2.660| 1.330| .665| .333| | | | | | | | | | | | | | |Venus | 6052| 6.602| 3.301| 1.650| .825| .413| .206| .103| | | | | | | | | | | |Moon | 1738| 1.896| .948| .474| .237| .118| .059| | | | | | | | | | | | |Mars | 3385| 3.692| 1.846| .923| .462| .231| .115| .058| |Phobos | 11| .012| | | | | | | |Deimos | 6| .007| | | | | | | | | | | | | | | | | |Io | 1821| 1.986| .993| .497| .248| .124| .062| | |Europa | 1565| 1.707| .854| .427| .213| .107| .053| | |Ganymede | 2634| 2.873| 1.437| .718| .359| .180| .090| | |Callisto | 2403| 2.621| 1.311| .655| .328| .164| .082| | | | | | | | | | | | |Mimas | 199| .217| | | | | | | |Enceladus| 249| .272| | | | | | | |Tethys | 523| .571| | | | | | | |Dione | 560| .611| | | | | | | |Rhea | 764| .833| .417| | | | | | |Iapetus | 718| .783| | | | | | | | | | | | | | | | | |Miranda | 236| .257| | | | | | | |Ariel | 579| .632| | | | | | | |Umbriel | 585| .638| | | | | | | |Titania | 789| .861| | | | | | | |Oberon | 761| .830| | | | | | | | | | | | | | | | | |Triton | 1353| 1.476| .738| .369| .184| .092| | | |_________|______|______|______|______|______|______|______|______| 6 - COMPILATION OF DIM'S The Mars Digital image models on this CDROM are compiled and archived in four stages or "levels", beginning with raw images. All of the corrections made during these stages have some level of uncertainty, so the processing sequence is designed to progress from corrections with the highest probability of accuracy to the lowest, and intermediate stages are preserved for future analytical use. Image processing software exists to perform the various stages of image correction and enhancement [18, 19]. 6.1 - LEVEL 1: RADIOMETRIC CORRECTION Level 1 processing includes removal of electronic shading, which is inherent in the imaging system, and artifacts such as minute dust specks on the vidicon tube, microphonic noise introduced by operation of other instruments on the spacecraft during imaging sequences, and data drop-outs and spikes [15]. Reseau marks are also located and removed during this stage; their precise locations are recorded for use during later geometric processing. A digital image label is created, containing the reseau-mark locations, geodetic control point and image tie-point locations, and a computed camera orientation matrix that will project the frame to a best-fit shape and position in a mosaic. Level 1 images have better resolution than those produced at any subsequent processing level. This is because they have not been resampled for geometric correction and projection; some loss of information is inevitable in any resampling, because the density values of multiple pixels and/or fractional pixels must be averaged to form new pixels in the output array. Photographic copies of Level 1 images, with spatial filter enhancement, are therefore the more useful photographic materials for visual interpretation. 6.2 - LEVEL 2: GEOMETRIC CORRECTION Level 2 processing includes removal of camera distortions and transformation from image to map coordinates in DM format according to parameters derived at the end of the Level 1 processing phase [20]. The resolution of each frame is preserved to some extent by oversampling in the output array; that is, by selecting a resolution step that results in an image with more lines and samples than the original image. Distortion corrections are based on preflight calibration of the reseau. Image transformation is based on camera orientation matrices derived by photogrammetric triangulation [21] modified as required for a best fit with adjacent images. On those images where matrices are not available, they are derived by matching corresponding points with images that have matrices. 6.3 - LEVEL 3: PHOTOMETRIC CORRECTION At level 3 processing apparent inconsistencies in surface brightness caused by variation in illumination geometry and by atmospheric effects are treated. Atmospheric scattering is a significant consideration on Mars. Different materials on any planet have different light-reflecting properties. Other photometric corrections are effective only to the extent that all geometric parameters can be modeled. In general, local topography is not included in the model (i.e., the surface model used is flat). Illumination geometry at each pixel, however, certainly depends on local topography; unless the topographic slope within a pixel is accurately known and compensated, the photometric correction cannot be perfect. All of these conditions are so complex that photometric correction of planetary images is likely to be only approximate for some time into the foreseeable future, although research into the effects and prototype examples of full three-dimensional treatment are now being pursued. An obvious example of the complexity of the problem would consist of a pair of images of the same landform illuminated from opposite directions. Only an extremely complex algorithm could accurately modify the shading in one of the images to match that of the other. No algorithm could restore detail lost in shadow. The photometric processing used in this MDIM was necessarily oversimplified, and incorporates spatial filtration that has the effect of subduing regional albedo markings. Further, quantitative radiometric analysis cannot be performed on the MDIM image collection. Prior to mosaicking, each of the digital images was first filtered to suppress regional scale albedo patterns larger than about one degree on the planet. This was done by applying a 251x251 high-pass boxcar filter [27] to each 16-bit integer image after geometric transformation. The high-pass filter was performed by convolving each image with a 251x251 boxcar of unit weight, subtracting this smoothed image from the original image and adding it to 16384. The value 16384 was added in order to keep pixel values always greater than zero. Each image was then linearly stretched and converted to 8-bit unsigned integer format. The stretch parameters were chosen to normalize the images so that features of the same relief in different images would have the same contrast although images were acquired with different incidence and emission angles and through different filters. The MINIMA (mapped to 0 in 8-bit unsigned integer space) and MAXIMA (mapped to 255 in 8-bit space) for the linear stretch were computed as follows: MINIMA = 16384 - DELTA MAXIMA = 16384 + DELTA DELTA= (Rf*EXP(-TAU*(MUc+MUoc))*SIN(Ic))/(EXP(-TAU*(MUs+MUos))*Sin(Is)) where MUc and MUoc are the cosines of the emission and incidence angles at the image center; TAU is the optical depth of the atmosphere; MUs and MUos are the emission and incidence angles the images are to be normalized to; and Rf is a scaling coefficient dependent on the spectral bandpass. The following were used for the entire series of images: MUs=0; MUos=45 degrees; TAU=0.3; Rf=1275 for red; Rf=815 for minus-blue, green, and clear; and Rf=435 for violet. If the value of MUoc was greater than 83 degrees, it was set to 83 degrees. Finally, a seam-removal technique was used in the mosaicking procedure. First the filtered and stretched images were mosaicked. This mosaic was then filtered with a 51x51 low-pass filter (convolved with a 51x51 boxcar of unit weight). A 51x51 high-pass filter was next applied to each individual image that makes up the mosaic. Finally, this mosaic of high-pass filtered images was added to the low-pass filtered mosaic. 6.4 - LEVEL 4: CONTROLLED MOSAICKING Compilation of an accurate digital mosaic (MDIM) of the entire surface of a planet is the final stage in the construction of a DIM. The MDIM is a digital image of the planet, with uniform resolution throughout. The resolution of level 2 images used in the compilation is compressed or expanded to match that of the MDIM. 6.5 - DENSITY CONTRAST OF MDIM IMAGES MDIM's data number (DN) dynamic range is designed so that every image file will match in contrast with any of its neighboring files. This allows image files to be mosaicked together without having obvious contrast boundaries in the mosaic. The highest contrast on Mars is in the polar region and the image density histograms of these areas fill the full dynamic range 0 to 255. There are image files that cover low-contrast areas on the planet and the density histograms of these images will fill only a limited part of the full 0 to 255 range. When viewing low-contrast images on a display device, the image will look very bland unless a contrast stretch is applied to the image values. For convenience of image display applications, the MDIM files have an object containing the image histogram in order to facilitate the rapid display of an image with optimum contrast. An image display application could extract the image histogram from the file, use it to determine an optimum contrast stretch, and then load the display's DN look-up table to perform the contrast stretch. 6.6 - MDIM IMAGE "ARTIFACTS" "Perfect" cartographic products do not exist--all compilations are compromises at some level and the MDIM digital cartographic products are no exception. Various artifacts exist in the MDIM products which require some explanation. The original Viking EDR images were mapped to the Sinusoidal projection using the simplest approach of resampling know as "nearest neighbor" interpolation. In this approach, the data value of the pixel in the input irregular image closest to the location of the pixel in the desired output image is used for the value of the output pixel. The intensity values resulting from this interpolation scheme correspond to true pixel locations that can be as much as the square root of two spacing in error. (A comparison of resampling interpolation schemes are described in Bernstein [22].) The minimum longitude limit of a file (keyword MINIMUM_LONGITUDE) does not always end on an even longitude boundary. This was intentional. There is an unresolved issue on how the ISO/CDROM standard defines the format of fixed length record files of odd size. To avoid this issue, the minimum longitude limit of the file was set to always insure there were an even number of pixels in the line length. Artifacts in the image data exist due to poorly interpolated reseau and improperly removed random noise and vidicon camera blemishes. 7 - CONCEPT OF THE TILING SCHEME Most MDIMs are far too large to be managed conveniently as single files. They must therefore be segmented by some scheme analogous to the segmentation of a map series into quadrangles. Mars has been segmented into 1964 quadrangles for high-resolution mapping at a scale of 1:500,000; this scheme has been selected for tiling the MDIM because it results in tiles of reasonably convenient size, and because it allows easy cross reference between the MDIM and published paper maps. The tiles have dimensions of 5 deg. latitude by 5 deg. longitude at the equator. The longitude dimension is modified to account for the convergence of meridians on a globe, beginning at 47.5 deg. latitude, so that each tile in the MDIM retains roughly the same area. Just as published maps are indexed by the latitude/longitude of their center points (truncated to the nearest integer degree to simplify the indexing), the labels of the files in the MDIM refer to the latitude/longitude of the center of the tile. Thus, the tile (MDIM file) labelled MI05N312 is centered on a point 5 deg. north of the equator and at a latitude of 312 deg. W. Each tile has its own central meridian in order to minimize the geometric distortion (shearing) of the data within the tile. Thus, each tile, with the exception of the tiles that make up the poles, can be independently displayed and its view will be quite reasonable with virtually no geometric distortion due to the nature of the projection. Thus, craters remain round rather than being oblong. With each tile having its own central meridian, simple display software can display a tile (or sub-area of a tile) with virtually no geometric distortion in the area of interest. One of the nice advantages of the Sinusoidal Equal-Area projection is the simple process for changing the central meridian of the projection. The central meridian is changed simply by sliding image lines parallel to one another (assuming nearest-neighbor interpolation). For a computer algorithm to convert an MDIM tile to a new central meridian, the algorithm need only calculate a starting offset (where to put the first sample of the input line) and simply move the pixels from the input buffer to the output buffer starting at the calculated offset. For example, if a feature of interest existed on a boundary between two tiles, it would be relatively simple to develop a program that would read the two tiles into memory, create an output memory array with a new central meridian equal to the boundary longitude between the two tiles, and then copy the input tile lines to the output tile lines with a calculated offset value for each line. 8 - FILES, DIRECTORIES, AND DISK CONTENTS 8.1 - IMAGE FILE NAMING CONVENTION Each image file has a unique name that is constructed according to the type of image file, resolution, and its central latitude and longitude. Because only eight characters are available as a file name, a highly compressed notation is used. The general form of an image file name is 'vwxxyzzz.IMG'. In this construct the 'v' field represents the type of image data in the file and it has three possible values: 'M' represents a MDIM image, 'T' represents a DTM image, and 'S' represents a shaded relief airbrush image. The 'w' field represents the resolution of the image file. In this field an alphabetic character is used to represent the scale: 'A' = 1/1 degrees/pixel, 'B' = 1/2, 'C' = 1/4, 'D' = 1/8, 'E' = 1/16, etc. For this volume set only the characters 'C' (1/4 degree/pixel), 'E' (1/16 degree/pixel), 'G' (1/64 degree/pixel), and 'I' (1/256 degree/pixel) are used. The 'xx' field is constructed from the central latitude of the image file. The central latitude is rounded down to the nearest whole integer of latitude. The 'y' field contains the value of 'N' for north latitude files, and 'S' for south latitude files. Finally, the 'zzz' field is constructed from the central longitude of the image file. The central longitude is rounded down to the nearest whole integer of longitude. The '.IMG' extension name is a PDS standard indicating the file is an image file. MI00N090.IMG is an example file name. It is an MDIM image, has a resolution of 1/256 degree/pixel, a center latitude at the equator, and a center longitude at 90 degrees. Table 2 summarizes the file naming convention for image files. TABLE 2. - IMAGE FILE NAME CONVENTION ------------------------------------- General Form of Image File Names - vwxxyzzz.IMG where: v = Type of image file M - Mars Digital Image Map T - Mars Digital Topographic Model S - Shaded Relief Airbrush Map w = Resolution code for image file C - 1/4 degrees/pixel E - 1/16 degrees/pixel G - 1/64 degrees/pixel I - 1/256 degrees/pixel xx = Central latitude value rounded down to nearest whole latitude y = North or South latitude N - North latitude S - South latitude zzz = Central longitude value rounded down to nearest whole longitude ------------------------------------------- 8.2 - DIRECTORIES The volume and directory structure of this CD-ROM conforms to the level-1 standard specified by the International Standards Organization (ISO). This standard is also known as the ISO-9660 standard. The ISO standard was used so that the disks can be accessed on a wide variety of computer systems. Information on the ISO-9660 CD-ROM standard is provided in Appendix A of this document. The Image files are subdivided into directories based on the type of image file (MDIM, DTM, shaded relief airbrush), the resolution of the image file, and for 1/256 and 1/64 degree/pixel image files the center latitude of the image. For 1/4 and 1/16 degree/pixel images the first two characters of a file name followed by six 'X' characters make up the directory name. For example, all of the 1/16 degree/pixel MDIM images are located in the directory MEXXXXXX (Volumes 1 and 6 only). For 1/256 and 1/64 degree/pixel images the first five characters of a file name followed by three 'X' characters make up the directory name. For example, all of the 1/256 degree/pixel images with center latitude 45 degrees north are located in the directory MI45NXXX. Volumes 1 and 6 contain a special directory called POLAR which contains Polar Stereographic projection images from 80 to 90 degrees latitude. The MDIM images, Shaded relief airbrush images, DTM image files (volume 7 only), supplemental files, documentation, and software are located in separate directories. Table 3 shows the contents of the 10 directories common to all seven volumes: 1) the root directory is the primary directory on the disk; 2) the DOCUMENT directory contains documentation files; 3) the GAZETTER directory contains the gazetteer table and supplemental files; 4) the INDEX directory contains the image index table; 5) the LABEL directory contains ancillary PDS label files; 6) the SOFTWARE directory contains supporting software for the MDIM image files; 7) the SCXXXXXX directory contains the 1/4 degree resolution shaded relief airbrush image; 8) the SEXXXXXX directory contains the shaded relief airbrush map at 1/16 degree resolution; and 9) the MEXXXXXX directory contains the 1/16 degree resolution MDIM images TABLE 3. - DIRECTORY CONTENTS OF COMMON DIRECTORIES --------------------------------------------------- root directory AAREADME.TXT - Introduction to this CDROM volume VOLDESC.SFD - Volume descriptor label DOCUMENT directory VOLINFO.TXT - Documentation for the MDIM volumes GAZETTER directory GAZETTER.TXT - Gazetteer documentation GAZETTER.LBL - PDS labels for the Gazetteer table GAZETTER.TAB - Gazetteer table for Mars GAZINFO.TXT - Information on Gazetter table GAZETTER.DBF - DBASE structure file for Gazetteer table WPMACRO.TXT - How do use the Word Perfect Macros *.WPM - Word Perfect macros for conversion of diacritical marks to Word Perfect format INDEX directory IMGINDEX.LBL - PDS labels for the image index table IMGINDEX.TAB - Image index table INDXINFO.TXT - Information on index table IMGINDEX.DBF - DBASE structure file for index table LABEL directory DSMAPDIM.LBL - Ancillary label for the data set map projection object. SOFTWARE directory MAC subdirectory - Macintosh display software PC subdirectory - IBM/PC display software SUN subdirectory - SUN Sparcstation display software VAX subdirectory - VAX/VMS Workstation display software SCXXXXXX directory SC00N000.IMG - Shaded Relief Airbrush image file at 1/4 degrees per pixel containing 1440 samples and 720 lines. SEXXXXXX directory *.IMG - Shaded Relief Airbrush image files at 1/16 MEXXXXXX directory *.IMG - MDIM image files at 1/16 degrees per pixel 9 - IMAGE FILE ORGANIZATION The record structure of the MDIM, Shaded Relief Airbrush, and DTM image files is a fixed-length format (see Appendix A for a description of fixed-length record files). There are three areas that make up the image file: the image label, the image histogram object, and the image object. 9.1 - IMAGE LABEL AREA The label area of a image file contains descriptive information about the image. The label consists of keyword statements that conform to version 2 of the Object Description Language (ODL) developed by NASA's PDS project [23, 24]. There are three types of ODL statements within a label: structural statements, keyword assignment statements, and pointer statements. Structural statements provide a shell around keyword assignment statements to delineate which data object the assignment statements are describing. The structural statements are: 1) OBJECT = object_name 2) END_OBJECT 3) END The OBJECT statement begins the description of a particular data object and the END_OBJECT statement signals the end of the object's description. All keyword assignment statements between an OBJECT and its corresponding END_OBJECT statement describe the particular object named in the OBJECT statement. The END statement terminates a label. A keyword assignment statement contains the name of an attribute and the value of that attribute. Keyword assignment statements are described in more detail in Appendix B of this document. These statements have the following format: name = value Values of keyword assignment statements can be numeric values, literals, and text strings. Pointer statements are a special class of keyword assignment statements. These pointers are expressed in the ODL using the following notation: ^object_name = location If the object is in the same file as the label, the location of the object is given as an integer representing the starting record number of the object, measured from the beginning of the file. The first label record in a file is record 1. Pointers are useful for describing the location of individual components of a data object. Pointer statements are also used for pointing to data or label information stored in separate files. An example of a detached label (i.e., label information stored in a separate file) is shown below: By convention, detached labels are found in the LABEL directory. ^STRUCTURE = 'logical_file_name' The value of 'logical_file_name' is the name of the detached label file containing the description. The keyword statements in the label are packed into the fixed-length records that make up the keyword label area. Each keyword statement is terminated by a carriage-return and line-feed character sequence. An example of a MDIM image label is shown in table 5. Descriptions of the keywords used in the MDIM label are found in Appendix C. TABLE 5. - EXAMPLE OF MDIM IMAGE LABEL -------------------------------------- CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL /* FILE FORMAT AND LENGTH */ RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 1184 FILE_RECORDS = 1283 LABEL_RECORDS = 2 /* POINTERS TO START RECORDS OF OBJECTS IN FILE */ ^IMAGE_HISTOGRAM = 3 ^IMAGE = 4 /* IMAGE DESCRIPTION */ DATA_SET_ID = "VO1/VO2-M-VIS-5-DIM-V1.0" SPACECRAFT_NAME = {VIKING_ORBITER_1, VIKING_ORBITER_2} TARGET_NAME = MARS IMAGE_ID = MI65N005 SOURCE_IMAGE_ID = {"793A03", "823A12", "669B17", "672B32", "672B55", "672B57", "672B58", "672B60", "672B61", "672B62", "672B83"} INSTRUMENT_NAME = {VISUAL_IMAGING_SUBSYSTEM_CAMERA_A, VISUAL_IMAGING_SUBSYSTEM_CAMERA_B} NOTE = "MARS DIGITAL IMAGE MAP, 1/256 DEG./PIXEL, CENTER LAT,LON 65.00, 5.000 " /* DESCRIPTION OF OBJECTS CONTAINED IN FILE */ OBJECT = IMAGE_HISTOGRAM ITEMS = 256 ITEM_TYPE = VAX_INTEGER ITEM_BITS = 32 END_OBJECT = IMAGE_HISTOGRAM OBJECT = IMAGE LINES = 1280 LINE_SAMPLES = 1184 SAMPLE_TYPE = UNSIGNED_INTEGER SAMPLE_BITS = 8 SAMPLE_BIT_MASK = 2#11111111# CHECKSUM = 123456789 END_OBJECT = IMAGE OBJECT = IMAGE_MAP_PROJECTION_CATALOG ^DATA_SET_MAP_PROJECTION_CATALOG = "DSMAPDIM.LBL" MAP_PROJECTION_TYPE = SINUSOIDAL MAP_RESOLUTION = 256 MAP_SCALE = 0.231352 MAXIMUM_LATITUDE = 67.50000 MINIMUM_LATITUDE = 62.50000 MAXIMUM_LONGITUDE = 10.00000 MINIMUM_LONGITUDE = -0.01627 X_AXIS_PROJECTION_OFFSET = -17280.000 Y_AXIS_PROJECTION_OFFSET = -591.038 A_AXIS_RADIUS = 3393.40 B_AXIS_RADIUS = 3393.40 C_AXIS_RADIUS = 3375.73 FIRST_STANDARD_PARALLEL = "N/A" SECOND_STANDARD_PARALLEL = "N/A" POSITIVE_LONGITUDE_DIRECTION = WEST CENTER_LATITUDE = 0.00000 CENTER_LONGITUDE = 5.00000 REFERENCE_LATITUDE = "N/A" REFERENCE_LONGITUDE = "N/A" X_AXIS_FIRST_PIXEL = 1 Y_AXIS_FIRST_PIXEL = 1 X_AXIS_LAST_PIXEL = 1280 Y_AXIS_LAST_PIXEL = 1184 MAP_PROJECTION_ROTATION = "N/A" END_OBJECT = IMAGE_MAP_PROJECTION_CATALOG END 9.2 - IMAGE HISTOGRAM OBJECT The first object after the label in an MDIM image file is the histogram of the image. The Image Histogram Object begins at the record specified by the ^IMAGE_HISTOGRAM pointer keyword. (Note, the first record in the file is defined as record 6.) The number of fixed-length records that make up the image histogram object can be determined by subtracting the value of the ^IMAGE pointer keyword from the ^IMAGE_HISTOGRAM pointer keyword value. These records, when concatenated together, contain the 256 elements of the image histogram with each element occupying four bytes. Each element is a 32-bit VAX integer [25]. The first element of the histogram contains the count of pixels in the image with the brightness value 0. The last element contains the count of pixels in the image with brightness value 255. 9.3 - IMAGE OBJECT The second object in the MDIM image file contains the image data. The image starts at the record specified by the ^IMAGE keyword. The number of records that make up the image is specified by the LINES keyword value. Each image line is stored in a separate fixed-length record. Each sample is an 8-bit unsigned integer as described by the SAMPLE_BITS and the SAMPLE_TYPE keywords in the label. For DTM data, each sample is a 16-bit signed integer. The LINE_SAMPLES keyword describes the number of elements in each image line. 10 - SOFTWARE 10.1 - SOFTWARE DISCLAIMER Although the software contained on the MDIM CDROMs have been used and tested, no warranty, expressed or implied, is made by NASA, the Jet Propulsion Laboratory (JPL), or the United States Geological Survey (USGS) as to the accuracy and functioning of the software and related materials, and no responsibility is assumed by NASA, JPL, or the USGS. 10.2 - SOFTWARE TOOLS Software is provided on the MDIM CDROMs to facilitate access to the image files. These files allow simple display capability of the individual image tiles. Software is provided for four hardware platforms: Apple Macintosh, IBM/PC, SUN Sparcstation, and VAX/VMS workstation systems. The software is located in subdirectories within the SOFTWARE directory. The subdirectories MAC (Macintosh software), PC (IBM/PC software), SUN (SUN Sparcstation), and VAX (VAX/VMS workstation) will be found. Within each subdirectory is located a file called SOFTINFO.TXT which describes how to use the software. 11 - IMAGE INDEX Each CDROM in the MDIM volume set contains an image index file (IMGINDEX.TAB) with catalog information about all MDIM image files in the collection. The image index file and it's associated PDS label file (IMGINDEX.LAB) are located in the INDEX directory. The catalog information in the index table includes the file names, CDROM volumes containing the MDIM image files, and mapping parameter information. The image index file has fixed-length records of length 512 bytes in ASCII character representation. Each record (row in the table) contains the information for a single MDIM image file. Table 6 describes the contents of the image index file located in the INDEX directory. All fields are in ASCII character format. The image index files are formatted to allow automatic data entry programs to access the data for entry into an existing data base system. The non-numeric fields are enclosed by double-quote characters. All fields are delimited by commas. The last two bytes in a record are carriage-control and line-feed characters. Table 6 gives the starting and ending byte positions of each field in the image index. These byte positions specify the actual fields and do not include the double-quote marks and commas that separate the fields. TABLE 6 - FORMAT OF IMAGE INDEX FILE ------------------------------------ Byte Positions Description ---------------------------------------------------------------------- 2 - 23 FILE_NAME: the fully qualified CDROM file name for the image file. The format of the directory name specification is the VAX/VMS directory format, with brackets indicating the directory hierarchy. Users on other systems will need to convert the directory names to the operating system formats. 26 - 35 MAXIMUM_LATITUDE: the Maximum latitude in the image file. Latitude ranges from +90.0 degrees for the north pole to -90.0 degrees for the south pole. 37 - 46 MINIMUM_LATITUDE: the minimum latitude in the image file. 48 - 58 MAXIMUM_LONGITUDE: the maximum longitude in the image file. Longitude ranges from +180.0 to -180 degrees. 60 - 70 MINIMUM_LONGITUDE: the minimum longitude in the image file. 72 - 82 CENTER_LONGITUDE: the center longitude of the Sinusoidal Equal-Area projection. 84 - 88 LINES: the number of lines in the image file. This parameter specifies the number of elements of the slowest varying dimension of the two dimensional image array. 90 - 94 LINE_SAMPLES: the number of samples in the image file. This parameter specifies the number of elements of the fastest varying dimension of the two dimensional image array. 96 - 99 MAP_RESOLUTION: the map resolution of the image file expressed as number of pixels per degree at the equator. 102 - 108 VOLUME_ID 1: list of CDROM volume ID's on which the image file 112 - 118 VOLUME_ID 2: is stored (VO_2001, VO_2002, etc.). There are 122 - 128 VOLUME_ID 3: seven fields in this list. Some image files at 132 - 138 VOLUME_ID 4: 1/256 and 1/64 degrees per pixel exist on more 142 - 148 VOLUME_ID 5: than one volume. The 1/4 and 1/16 degree per pixel 152 - 158 VOLUME_ID 6: MDIM image files exist on all seven volumes. 162 - 168 VOLUME_ID 7: 171 - 181 X_AXIS_PROJECTION_OFFSET: the line position of line 1.0 and sample 1.0 in the x,y coordinates relative to the origin of the map projection. 183 - 193 Y_AXIS_PROJECTION_OFFSET: the sample position of line 1.0, sample 1.0 in the x,y coordinates relative to the origin of the map projection. 196 - 271 NOTE: contains a brief description of the image file. The field indicates the number of degrees/pixel of the file and specifies the center latitude and longitude coordinate of the image file. 275 - 282 IMAGE_ID: this field contains a six-character string to identify the image file. The field matches the name of the input file. 285 - 295 MAP_SCALE: this field gives the number of kilometers per pixel at the equator. 298 - 303 SOURCE_IMAGE_ID 1: List of up to 20 source images. This list 307 - 312 SOURCE_IMAGE_ID 2: contains the image identifiers of the 316 - 321 SOURCE_IMAGE_ID 3: Viking Orbiter images that were used to 325 - 330 SOURCE_IMAGE_ID 4: create the MDIM image file. Blank fields 334 - 339 SOURCE_IMAGE_ID 5: indicate no SOURCE_IMAGE_ID. There are 343 - 348 SOURCE_IMAGE_ID 6: never more than 20 images that make up 352 - 357 SOURCE_IMAGE_ID 7: an MDIM image file. 361 - 366 SOURCE_IMAGE_ID 8: 370 - 375 SOURCE_IMAGE_ID 9: 379 - 384 SOURCE_IMAGE_ID 10: 388 - 393 SOURCE_IMAGE_ID 11: 397 - 402 SOURCE_IMAGE_ID 12: 406 - 411 SOURCE_IMAGE_ID 13: 415 - 420 SOURCE_IMAGE_ID 14: 424 - 429 SOURCE_IMAGE_ID 15: 433 - 438 SOURCE_IMAGE_ID 16: 442 - 447 SOURCE_IMAGE_ID 17: 451 - 456 SOURCE_IMAGE_ID 18: 460 - 465 SOURCE_IMAGE_ID 19: 469 - 474 SOURCE_IMAGE_ID 20: 12 - GAZETTEER Planetary nomenclature, like terrestrial nomenclature, is used to uniquely identify a feature on the surface of a planet or satellite so that the feature can be easily located, described, or discussed. The gazetteer on the MDIM CDROM volume set contains detailed information about all named features on Mars that the International Astronomical Union (IAU) has named and approved from its founding in 1919 through its triennial meeting in 1991. The gazetteer is located on each CDROM volume of the seven volume set. The information pertinent to the gazetteer is located in the GAZETTER directory (a letter 'E' has been intentionally dropped from the spelling of gazetteer because of the eight character limit on file names on IBM/PC systems). In this directory, a detailed documentation of the gazetteer can be found in the GAZETTER.TXT file. The file GAZETTER.TAB is the gazetteer table. Each row in the table contains the description of a single Martian feature. Finally, the GAZETTER.LBL file is the PDS detached label that describes the file contents and format. Ancillary files exist in the GAZETTER directory for users of the Word Perfect text editor. The diacritical marks located in the gazetteer can be converted to the Work Perfect format with the macros included in this directory. Files which end in the extension *.WPM contain the Word Perfect Macros. The document file WPMACRO.TXT describes the use of these macros. 13 - ACKNOWLEDGEMENTS The National Aeronautics and Space Administration is charged with the responsibility for coordination of a program of systematic exploration of the planets by U.S. spacecraft. To this end, it finances spaceflight missions and data analysis and research programs administered and performed by numerous institutions. The Geological Survey of the U.S. Department of the Interior is the agency that performs most of the mapping in support of NASA's program of planetary exploration and scientific research. The digital Mars maps contained in these volumes were compiled by the U.S. Geological Survey (USGS) under funding provided by NASA through its Geology and Geophysics Program at NASA headquarters, Washington, DC, and through the Mars Observer Project administered by the Jet Propulsion Laboratory, Pasadena, California. NASA's Planetary Data System provided the guidance and standards required to manufacture and distribute the optical disks containing this MDIM and DTM of Mars. Compilation of the Mars digital models was performed at USGS under the direction of R.M. Batson, L.A. Soderblom, and Sherman S.C. Wu, Principal Investigator and Co-Investigators, respectively. Kathleen Edwards provided the technical management and supervision of a team of 14 technicians who compiled the MDIM. The design, layout, and production of the CDROMs was performed by E.M. Eliason and A. Manley at the USGS, and M. Martin and J. Hyon at JPL 14 - REFERENCES 1. Snyder, C. W., 1977. The Missions of the Viking Orbiters, J. Geophys. Res., 82, p. 3971-3983. 2. Snyder, C. W., 1979. The Extended Mission of Viking, J. Geophys. Res., 84, p. 7917-7933. 3. Benesh, M., and T. Thorpe, 1976. Viking Orbiter 1975 visual imaging subsystem calibration report, JPL Document 611-125, Jet Propulsion Laboratory, Pasadena, Ca. 4. Klaasen, K. P., T. E. Thorpe, and L. A. Morabito, 1977. Inflight performance of the Viking visual imaging subsystem, Applied Optics, 16, p. 3158-3170. 5. Wellman, J. B., F. P. Landauer, D. D. Norris, and T. E. Thorpe, The Viking Orbiter visual imaging subsystem, J. Spacecr. Rockets, 13, p. 660-666, 1976. 6. Batson, R.M., 1987, Digital cartography of the planets: new methods, its status, and its future: Photogrammetric Engineering and Remote Sensing, vol. 53, no. 9, p. 1211-1218. 7. Batson, R.M., 1990, Cartography, in Greeley, Ronald, and Batson, R.M., eds, Planetary Mapping: New York, Cambridge University Press, p. 60-95. 8. Batson, R.M., 1990, Appendix I: Map formats and projections used in planetary cartography, in Greeley, Ronald, and Batson, R.M., eds, Planetary Mapping: New York, Cambridge University Press, p. 261-276. 9. Batson, R.M., 1990, Appendix III: Digital planetary cartography, in Greeley, Ronald, and Batson, R.M., eds, Planetary Mapping: New York, Cambridge University Press, p. 289-287. 10. Wu, S.S.C., and Schafer, F.J., 1984, Mars control network: American Society of Photogrammetry, in Technical papers of the 50th annual meeting of the American Society of Photogrammetry, vol. 2, Washington, D.C., March 11 - 16, 1984, p. 456-463. 11. Clark, David, 1923. Plane and geodetic surveying for engineers, vol. 2, higher surveying [5th Ed., 1963, Revised by Clendinning, James]. Constable & Co. Ltd., London, p. 599-600. 12. Johnson, T.V., L.A. Soderblom, J.A. Mosher, G.E. Danielson, A.F. Cook, and P. Kupferman, 1983. Global multispectral mosaics of the icy Galilean satellites. Journal of Geophysical Research, vol. 88, no. B-7, p. 5789-5805. 13. Kieffer, H.H., P.A. Davis, and L.A. Soderblom, 1981. Mars' global properties: Maps and applications. Proceedings of Lunar and Planetary Science Conference XII, Houston, Texas, March 16-20, 1981, p. 1395-1417. 14. Pettengill, Gordon H., Donald B. Campbell, and Harold Masursky, 1980. The surface of Venus. Scientific American, vol. 243, no. 2, p. 54-65. 15. Soderblom, L.A., Kathleen Edwards, E.M. Eliason, E.M. Sanchez, and M.P. Charette, 1978. Global color variations on the Martian surface. Icarus, vol. 34, p. 446-464. 16. Snyder, J. P., 1982. Map projections used by the U.S. Geological Survey. Geological Survey Bulletin 1532, U.S. Government Printing Office, Washington D.C., 313 p. 17. Planetary Cartography Working Group, 1984. Planetary cartography in the next decade (1984-1994). National Aeronautics and Space Administration Special Publication 475, 71 p. 18. LaVoie, S., C. Avis, H. Mortensen, C. Stanley, and L. Wainio, 1987. VICAR - User's Guide, JPL Document D-4186, Jet Propulsion Laboratory, Pasadena, Ca. 19. Planetary Image Cartography System (PICS), Unpublished Manual, Branch of Astrogeology, U. S. Geological Survey, Flagstaff, Az., 1987. PICS is an integrated computerized system for the systematic reduction, display, mapping, and analysis of planetary image data. 20. Edwards, Kathleen, 1987, Geometric processing of digital images of the planets: Photogrammetric Engineering and Remote Sensing, vol. 53, no. 9, p. 1219-1222. 21. Wu, S.S.C., and Doyle, F.J., 1990, Topographic mapping, in Greeley, Ronald, and Batson, R.M., eds, Planetary Mapping: New York, Cambridge University Press, p. 169-207. 22. Bernstein, R., Branning, H., 2nd Ferneyhough, D.G., 1971, Geometric and radiometric correction of high resolution images by digital image processing techniques; IEEE Intl. Geosci. Electronics Symp., Washington, D.C. 23. Davis, R. L., June 1990. Specification for the Object Description Language, Version 2.0; Available from the PDS, Jet Propulsion Laboratory, Pasadena, Ca. 24. Cribbs, M., and Wagner, D., May, 1991. Planetary Data System Data Preparation Workbook; vols. 1 and 2, JPL Document 7669, Jet Propulsion Laboratory, Pasadena, Ca. 25. VAX integers, as storage units in data files, are configured in "least significant byte first" order. This is the order for integer values used by VAX and IBM PC computer systems. Users of other computer architectures (IBM Mainframes, Macintosh, SUN, and Apollo) may need to swap the high and low byte positions for 16-bit integer data. For 32-bit integer data, swap byte pairs 1 and 4, and 2 and 3. For example, hexadecimal value AA BB CC DD becomes DD CC BB AA. 26. Information processing -- Volume and file structure of CDROM for information interchange, ISO/DIS document number 9660, International Organization for Standardization, 1 Rue de Varembe, Case Postale 56, CH-1121 Geneva 20, Switzerland, 1987. 27. McDonnell, M. J., 1981. Box-filtering Techniques: Computer Graphics and Image Processing, Vol. 17, pp 65-70. APPENDIX A - ISO VOLUME AND DIRECTORY STANDARD A.1 - VOLUME AND DIRECTORY STRUCTURES The volume and directory structure of the CDROM conforms to the standard specified by the International Organization for Standardization (ISO) [26]. This standard is known as the ISO-9660 standard. This CDROM disk conforms to the first level of interchange, level-1. A.2 - FILE STRUCTURE The files on this CDROM are of two types: fixed-length record files, and stream format files. The characteristics of each record type are described in the following sections. A.2.1 - FIXED LENGTH RECORDS Records in a file with fixed-length records are all the same length, and there is no embedded information to indicate the beginning or end of a record. Fixed-length records allow any part of a file to be accessed directly without the need to pass through the file sequentially. Image files with the file name extension '.IMG' and table files with the file name extension '.TAB' are fixed-length record files. The starting byte of any record can be calculated as follows: offset = (record-1)*length where: offset = offset byte position of record from start of file record = desired record to access length = length of record in bytes A.2.3 - STREAM FILES Stream files typically are used to store ASCII text such as documentation and program source code. A stream file may have records of varying lengths. The end of a record is marked by two bytes containing the ASCII carriage return and line feed characters (hex 0D and 0A). Stream files are different from variable-length record files, which store the record size in the first two bytes of each record. On this CDROM, the documentation files, detached label files, and software source code files are in stream format. They may be printed or displayed on a terminal. Their file names have the extensions '.TXT', '.LBL', '.C', or '.FOR'. A.2.4 - EXTENDED ATTRIBUTE RECORD An extended attribute record (XAR) contains information about a file's record format, record attributes, and record length. The extended attribute record is not considered part of the file and is not seen by programs accessing a file with high-level I/O routines. Not all computer operating systems support extended attribute records. Those that do not will simply bypass the XAR when accessing a file. APPENDIX B - SYNTACTIC RULES OF KEYWORD ASSIGNMENT STATEMENTS The label area of the image files use the syntactic rules of the Object Definition Language (ODL) adopted by the PDS. This appendix provides only a very brief description of the syntax of the ODL language. For a complete description of the ODL language see Davis [23], and the Planetary Data System Data Preparation Workbook - Volume 1 [24]. A keyword assignment statement, made up of a string of ASCII characters, contains the name of an attribute and the value of that attribute. A keyword assignment statement has the general form shown below: name = value [/* comment */] The format of each keyword assignment statement is essentially free-form; blanks and tabs are typically ignored by a parsing routine. An attribute name is separated from its value by the equal symbol (=). Each keyword assignment statement may optionally be followed by a comment that more completely describes the entry. The comment begins with a slash character followed by an asterisk character (/*), and terminates with an asterisk character followed by a slash character (*/). Comments may also exist on a line without a keyword assignment statement. Note that the brackets indicate that the comment and its delimiter are optional. The MDIM labels have carriage-return and line-feed characters following the "name = value" sequence. The ODL language does not require a cr/lf sequence at the end of each keyword assignment statement but are optional in order to facilitate printing of keywords. Values associated with an attribute can be integers, real numbers, unitized real numbers, literals, times, or text strings. B.1 - INTEGER NUMBERS An integer value consists of a string of digits preceded optionally by a sign (+ or -). Non-decimal based integers are expressed according to the Ada language convention: b#nnnnnnn#, where 'b' represents the base of the number, and '#' delimits the number 'nnnnnnnn'. For example, the number expressed as 2#111# represents the binary number 111, which is 7 in base 10. B.2 - REAL NUMBERS A real number has the form: [s]f.d[En] where: s = optional sign (+ or -) f = one or more digits that specify the integral portion of the number. d = one or more digits that specify the fraction portion of the number. n = an optional exponent expressed as a power of 10. A unitized real number is a real number with an associated unit of measurement. The units for a real number value are enclosed in angle brackets (< >). For example, 1.234 indicates a value of 1.234 seconds. B.3 - DATES AND TIMES A special form of a numeric field is a time value. The following format of date/time representations is used: yyyy-mm-ddThh:mm:ss.fffZ where: yyyy = year mm = month dd = day of month hh = hour mm = minute ss = seconds fff = fraction of a second Z = The Z qualifier indicates the time is expressed as Universal Time Corrected (UTC). B.4 - LITERAL VALUES A literal value is an alphanumeric string that is a member of a set of finite values. It can also contain underscore character (_). A literal value must be delimited by double or single quote characters (" or ') if it does not begin with a letter (A-Z). If the literal begins with a letter, it does not have to be enclosed in single quotes. If a literal appears within quotes, the literal may contain any printable ASCII character. For example, the literal value "1:1" is legal as long as the single or double quoted format is used. A keyword assignment statement using a literal value might look like the examples shown below: DATA_SET_ID = "VO1/VO2-M-VIS-5-DIM-V1.0" TARGET_NAME = MARS B.5 - TEXT CHARACTER STRINGS Text strings can be any length and can consist of any sequence of printable ASCII characters including tabs, blanks, carriage-control, or line-feed characters. Text strings are enclosed in double quote characters. If the text string comprises several lines, it continues until a double quote character is encountered and includes the carriage- control and line-feed characters. APPENDIX C - KEYWORD ASSIGNMENTS FOR MDIM IMAGES CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL This keyword provides a mechanism for image files on this CDROM to conform to the SFDU (Standard Formatted Data Unit) convention. The first 20 bytes identify the file as a CCSDS SFDU entity. The next 20 bytes identify the file as a registered product of the JPL SFDU control authority. The components of both SFDU labels are the control authority identifier (characters 1-4), the version identifier (character 5), the class identifier (character 6), a spare field (characters 7-8), a format identifier (characters 9-12), and a length field indicator (characters 13-20). The version identifier indicates a "Version-3" label, which allows files to be delimited by an end-of-file marker, rather than requiring a byte count to be embedded in the label. The keyword conforms to standard PDS keyword syntax and the value associated with this keyword will always be SFDU_LABEL. RECORD_TYPE = FIXED_LENGTH This keyword defines the record structure of the file. The MDIM image files are always fixed-length record files. This keyword always contains the value FIXED_LENGTH. RECORD_BYTES = xxxx Record length in bytes for fixed length records. FILE_RECORDS = xxxx Total number of records contained in the file. LABEL_RECORDS = xxxx Number of records in the label area of the image file. ^IMAGE_HISTOGRAM = xx The (^) character prefixing a keyword indicates that the keyword is a pointer to the starting record of a data object in the file. In this case, the keyword is the pointer to the Image Histogram Object. The keyword value indicates the starting record in the file for the Image Histogram Object. The number of records found in an object is determined by differencing the value of the pointer keyword from the value of the next pointer. ^IMAGE = xx The keyword value points to the starting fixed-length record in the file for the Image Object. DATA_SET_ID = "VO1/VO2-M-VIS-5-DIM-V1.0" The PDS defined data set identifier for the MDIM image data products produced from the Viking Orbiter Imaging System. SPACECRAFT_NAME = {VIKING_ORBITER_1, VIKING_ORBITER_2} The spacecraft name identifies the spacecrafts that acquired the image data. For the MDIM images, this keyword always contains the values VIKING_ORBITER_1 and VIKING_ORBITER_2 to indicate that images that make up the mosaics are a composite of data acquired from these two spacecraft. TARGET_NAME = MARS Observation target of the image. This value is always MARS for the MDIM digital image products. IMAGE_ID = vwxxyzzz This is the unique image identification code for the MDIM image. The IMAGE_ID is the same as the name given to the file. v = Type of image file M - Mars Digital Image Map T - Mars Digital Topographic Model S - Shaded Relief Airbrush Map w = Resolution code for image file C - 1/4 degrees/pixel E - 1/16 degrees/pixel G - 1/64 degrees/pixel I - 1/256 degrees/pixel xx = Central latitude value rounded down to nearest whole latitude y = North or South latitude N - North latitude S - South latitude zzz = Central longitude value rounded down to nearest whole longitude SOURCE_IMAGE_ID = {"316A27","427A33"} The MDIM images are a mosaic of Viking Orbiter images. This keyword lists the IMAGE_ID's of those Viking Orbiter images that were used in the mosaic for this image. This keyword is a set of literal values. INSTRUMENT_NAME = {VISUAL_IMAGING_SUBSYSTEM_CAMERA_A, VISUAL_IMAGING_SUBSYSTEM_CAMERA_B} The name of the cameras used to acquire the image. This keyword will always contain the values VISUAL_IMAGING_SUBSYSTEM_CAMERA_A and _B. NOTE = "description" This field provides the product name, scale, and latitude and longitude of the center of the image. OBJECT = IMAGE_HISTOGRAM ITEMS = 256 ITEM_TYPE = VAX_INTEGER ITEM_BITS = 32 END_OBJECT = IMAGE_HISTOGRAM This keyword sequence identifies the Image Histogram Object. The object contains 256 elements, stored in VAX integer format [22]. Each element has 32 bits. The records associated with an object are concatenated together to make the object. Some objects do not completely fill the records that make up the object. OBJECT = IMAGE LINES = xxxx LINE_SAMPLES = xxxx SAMPLE_TYPE = UNSIGNED_INTEGER SAMPLE_BITS = 8 SAMPLE_BIT_MASK = 2#11111111# CHECKSUM = xxxxxxxxx END_OBJECT = IMAGE This keyword sequence describes the image object. The meaning of the keywords with this sequence area as follows: LINES = xxxx Number of image lines in the image object. LINE_SAMPLES = xxxx Number of samples in each image line. SAMPLE_TYPE = UNSIGNED_INTEGER Data type for pixels values, should always be unsigned integers. For DTM images the value is SIGNED_INTEGER SAMPLE_BITS = 8 Number of bits in a pixel, which are 8-bit values in the range 0 to 255. For DTM images the value is 16 SAMPLE_BIT_MASK = 2#11111111# Active bits in an image sample. The number is expressed as a base 2 value in the Ada language number base convention. The keyword value consists of a string of 1's or 0's. The value 1 indicates a bit is active and a 0 indicates a bit is not in use. For example, SAMPLE_BIT_MASK = 2#11111111# indicates all bits active. CHECKSUM = xxxxxxxxxx The sum of all the pixel values within the image. This parameter can be used to verify the reading of an image file. OBJECT = IMAGE_MAP_PROJECTION_CATALOG ^DATA_SET_MAP_PROJECTION_CATALOG = "DSMAPDIM.LBL" MAP_PROJECTION_TYPE = SINUSOIDAL MAP_RESOLUTION = x MAP_SCALE = x.xxxxx MAXIMUM_LATITUDE = x.xxxxx MINIMUM_LATITUDE = x.xxxxx MAXIMUM_LONGITUDE = x.xxxxx MINIMUM_LONGITUDE = x.xxxxx X_AXIS_PROJECTION_OFFSET = x.xxxxx Y_AXIS_PROJECTION_OFFSET = x.xxxxx A_AXIS_RADIUS = 3393.40 B_AXIS_RADIUS = 3393.40 C_AXIS_RADIUS = 3375.73 FIRST_STANDARD_PARALLEL = "N/A" SECOND_STANDARD_PARALLEL = "N/A" POSITIVE_LONGITUDE_DIRECTION = WEST CENTER_LATITUDE = 0.00000 CENTER_LONGITUDE = x.xxxxx REFERENCE_LATITUDE = "N/A" REFERENCE_LONGITUDE = "N/A" X_AXIS_FIRST_PIXEL = 1 Y_AXIS_FIRST_PIXEL = 1 X_AXIS_LAST_PIXEL = xxxx Y_AXIS_LAST_PIXEL = xxxx MAP_PROJECTION_ROTATION = "N/A" END_OBJECT = IMAGE_MAP_PROJECTION_CATALOG This keyword sequence describes the cartographic keywords that define the mapping parameters of the image. ^DATA_SET_MAP_PROJECTION_CATALOG = "DSMAPDIM.LBL" This keyword points to a separate file (DSMAPDIM.LBL) on the CDROM that contains supplemental and nonessential keyword descriptors for map projection parameters. By convention, supplemental labels are found in the LABEL directory. MAP_PROJECTION_TYPE = SINUSOIDAL This element identifies the type of projection used in the map. This value is always SINUSOIDAL for the MDIM products and signifies a Sinusoidal Equal-Area projection. MAP_RESOLUTION = x This element identifies the scale of the MDIM image file. The resolution is defined in pixels per degree. MAP_SCALE = x.xxxxx This element identifies the scale of the MDIM image file and is defined in kilometers per pixel. MAXIMUM_LATITUDE = x.xxxxx This element specifies the northern most latitude in the MDIM image file. MINIMUM_LATITUDE = x.xxxxx This element specifies the southern most latitude in the MDIM image file. MAXIMUM_LONGITUDE = x.xxxxx This element specifies the left-most longitude of the image file. MINIMUM_LONGITUDE = x.xxxxx This element specifies the right-most longitude of the image file. X_AXIS_PROJECTION_OFFSET = x.xxxxx This element provides the line offset value of the map projection origin position from line and sample 1,1. Note that the positive direction is to the right and down. See Appendix E for the use of this element. Y_AXIS_PROJECTION_OFFSET = x.xxxxx This element provides the sample offset value of the map projection origin position from line and sample 1,1. Note that the positive direction is to the right and down. See Appendix E for the use of this element. A_AXIS_RADIUS = 3393.40 B_AXIS_RADIUS = 3393.40 C_AXIS_RADIUS = 3375.73 These elements provide the semi-major axis (A), intermediate axis (B), and semi-minor axis of the ellipsoid that defines the shape of the body defined in kilometers. These values are always 3393.40, 3393.40, and 3375.73 respectively. FIRST_STANDARD_PARALLEL = "N/A" This element is a mapping transformation parameter. The Sinusoidal Equal-Area projection does not used this element. SECOND_STANDARD_PARALLEL = "N/A" This element is a mapping transformation parameter. The Sinusoidal Equal-Area projection does not used this element. POSITIVE_LONGITUDE_DIRECTION = WEST This element identifies the direction of longitude (EAST,WEST) for a planet. The IAU definition for direction of positive longitude is adopted. For MARS this direction is WEST. CENTER_LATITUDE = 0.00000 This element identifies the center latitude of the projection. For Sinusoidal Equal-Area projections, this value is zero. CENTER_LONGITUDE = x.xxxxx This element identifies the center longitude of the projection. Each MDIM image file has its own center longitude. See Appendix E for the use of this mapping parameter. REFERENCE_LATITUDE = "N/A" This element is a mapping transformation parameter. The Sinusoidal Equal-Area projection does not used this element. REFERENCE_LONGITUDE = "N/A" This element is a mapping transformation parameter. The Sinusoidal Equal-Area projection does not used this element. X_AXIS_FIRST_PIXEL = 1 This element provides the x-dimension index to be assigned the first pixel that was physically recorded at the beginning of the image array. This value always 1 for MDIM image files. Y_AXIS_FIRST_PIXEL = 1 This element provides the y-dimension index to be assigned the first pixel that was physically recorded at the beginning of the image array. This value always 1 for MDIM image files. X_AXIS_LAST_PIXEL = xxxx This element provides the x-dimension index to be assigned the last pixel that was physically recorded at the end of the image array. For MDIM image files, this element equals the number of lines in the image. Y_AXIS_LAST_PIXEL = xxxx This element provides the y-dimension index to be assigned the last pixel that is physically recorded at the end of the image array. For MDIM image files, this element equals the number of samples in the image. MAP_PROJECTION_ROTATION = "N/A" This element is a mapping transformation parameter. The Sinusoidal Equal-Area projection does not used this element. END The keyword entries with a line that contains only the word END. Bytes in the label area after the END statement are ignored. APPENDIX D - GEOMETRIC DEFINITION OF A PIXEL The purpose here is to describe the spatial or geometric definition of a pixel used in the generation and utilization of the MDIM digital image products. A broad range of factors enters into this question. For example, is a pixel to be conceived of as a point or as an area? The point definition would be most convenient, for instance, when dealing with coordinate grid overlays. This results in an odd number of pixels across a map that has an even number of spatial increments. For changing scales (for instance by even powers of 2) this definition becomes a problem. In this case it makes more sense to treat a pixel as a finite area. Then an even number of pixels covers an even number of spatial increments and decreasing/increasing scales by a power of 2 becomes trivial. However, grids now fall between pixels, at least in a mathematical sense. Their treatment in the generation of hardcopy therefore becomes an issue. It was decided that the area concept of a pixel was the better choice; we would have to live with the asymmetries introduced in things like cartographic grids. There are various solutions: (1) use two pixels for the width of a grid line, (2) stagger grid pixels back-and-forth across the mathematical position, (3) use a convention whereby grid lines are systematically drawn offset from their mathematical position. The next issue is the conversion between integer coordinates and real coordinates of the pixel mesh. We adopt the convention that pixels are numbered (or named if you like) beginning in the upper left corner with line 1, sample 1 (pixel 1,1); lines increase downward; samples increase to the right. (Even this is not a universal standard; some astronomical systems begin, perhaps more logically, in the lower left corner.) There are three reasonable possibilities for aligning a real, or floating point, coordinate system with the pixel mesh: the coordinate 1.0, 1.0 could be the upper left, the center, or the lower right of pixel 1,1. The convention historically used for geometric calibration files (reseau positions) and also used in the Multimission Image Processing Laboratory at the Jet Propulsion Laboratory, is that the center of the pixel is defined as its location in real coordinates. In other words, the real coordinates of the center of pixel 1,1 are 1.0, 1.0. The top left corner of the pixel is .5, .5 and the bottom right corner is 1.49999..., 1.499999. The bottom and right edge of a pixel is the mathematically open boundary. This is the standard adopted in the MDIM image products. Cartographic conventions must also be defined. The map projection representation of a pixel is mathematically open at the increasing (right and lower) boundaries, and mathematically closed at its left and upper boundaries. An exception occurs at the physical limits of the projection; the lower boundary of the lowest pixel is closed to include the limit of the projection (e. g. the south pole). Figure D.1 shows the coordinates of Pixel 1,1. Figure D.1 - Coordinates of Pixel 1,1 longitude 180.0 179.00001 | | latitude | | line 90.0 -- ----------------- -- .5 | | | | | | | | | + | | (1.0,1.0) | | | | | | | 89.00001 -- ----------------- -- 1.49999 | | | | sample .5 1.49999 Finally, we must select a convention for drawing grid lines for various cartographic coordinates on planetary images and maps. The convention used for MDIM image products is that a grid line is drawn in the pixels that contain its floating point value until the open boundary is reached and then an exception is made so that the outer range of latitude and longitude will always appear on the image. This means, in the example given above, a 10 degree grid would start on pixel 1 and be drawn on every tenth pixel (11,21,31,...) until the open boundary is reached. Then the line would be drawn on the pixel previous to the open boundary (line 180 instead of line 181, or sample 360 instead of 361). To summarize, the MDIM conventions are: 1) Pixels are treated as areas, not as points. 2) The integer coordinates begin with 1,1 (read "line 1, sample 1") for the upper-left-most pixel; lines increase downward; samples increase to the right. 3) Integer and floating point image coordinates are the same at the center of a pixel. 4) Grids will be drawn in the pixels that contain the floating point location of the grid lines except for open boundaries, which will be drawn to the left or above the open boundary. APPENDIX E - SINUSOIDAL EQUAL-AREA PROJECTION EQUATION MDIM's are presented in a Sinusoidal Equal-area map projection. In this projection, parallels of latitude are straight lines, with constant distances between equal latitude intervals. Lines of constant longitude on either side of the projection meridian are curved since longitude intervals decrease with the cosine of latitude to account for their convergence toward the poles. This projection offers a number of advantages for storing and managing global digital data; in particular, it is computationally simple, and data are stored in a compact form. The Sinusoidal Equal-area projection is characterized by a projection longitude, which is the center meridian of the projection, and a scale, which is given in units of pixels/degree. The center latitude for all MDIM's is the equator. Each MDIM file contains its own central meridian. The transformation from latitude and longitude to line and sample for planets with a direction of positive longitude of WEST is given by the following equations: line = INT(X_AXIS_PROJECTION_OFFSET - lat*MAP_RESOLUTION + 1.0) sample = INT(Y_AXIS_PROJECTION_OFFSET - (lon - CENTER_LONGITUDE)* MAP_RESOLUTION*cos(lat) + 1.0) Note that integral values of line and sample correspond to center of a pixel. Lat and lon are the latitude and longitude of a given spot on the surface. INT is the fortran equivalent floating point to integer function. This function converts floating point values to integer by truncation of the fractional part of the floating point value. X_AXIS_PROJECTION_OFFSET is the line number minus one on which the map projection origin occurs. The map projection origin is the intersection of the equator and the projection longitude. The value of X_AXIS_PROJECTION_OFFSET is positive for images starting north of the equator and is negative for images starting south of the equator. The X_AXIS_PROJECTION_OFFSET is found in the labels of each image file. Y_AXIS_PROJECTION_OFFSET is the nearest sample number to the left of the projection longitude. The value of Y_AXIS_PROJECTION_OFFSET is positive for images starting to the west of the projection longitude and is negative for images starting to the east of the projection longitude. The Y_AXIS_PROJECTION_OFFSET is found in the labels of each image file. CENTER_LONGITUDE is the value of the projection longitude, which is the longitude that passes through the center of the projection. The CENTER_LONGITUDE is found in the labels of each image file. MAP_RESOLUTION is the number of pixels per degree on the planet. The values for MDIM products will be 256, 64, 16, and 4. The MAP_RESOLUTION is found in the labels of each image file. There are four PDS parameters that specify the latitude and longitude boundaries of an image. MAXIMUM_LATITUDE and MINIMUM_LATITUDE specify the latitude boundaries of the image, and MAXIMUM_LONGITUDE and MINIMUM_LONGITUDE specify the longitudinal boundaries of the map. A special note is required for the MAXIMUM_LONGITUDE and MINIMUM_LONGITUDE parameters that define the boundaries of an image. The MAXIMUM_LONGITUDE will be greater than the MINIMUM_LONGITUDE except when the image map crosses the zero meridian. When the zero meridian is contained in the image area, then the MINIMUM_LONGITUDE will be greater than the MAXIMUM_LONGITUDE. When this occurs, it may be convenient for a computer algorithm that uses these parameters to subtract 360.0 degrees from the MINIMUM_LONGITUDE. For example, if an image had longitude boundary limits from 10.0 degrees longitude (MAXIMUM_LONGITUDE) to 350.0 degrees longitude (MINIMUM_LONGITUDE) then it is implied that the zero meridian is in the middle of the image file. One could think of the longitude limits of the file going from 10.0 to -10.0 degrees longitude. For global maps that cover the entire 360 degrees of a planet, the MINIMUM_LONGITUDE will equal the MAXIMUM_LONGITUDE indicating that the "left" edge of the map has the same longitude as the "right" edge of the map.