From: "Randolph L Kirk"
Subject: Feedback solicited on format of future USGS Mars products Dear Colleague: The purpose of this message is to describe a series of new Mars cartographic datasets that will be produced by the USGS over the next several years and to solicit your feedback on various aspects of the design of these products that represent departures from our past and present Mars mosaics. To begin with, we are pleased to announce that we have completed the first of several planned revisions of the global digital image mosaic of Mars, or MDIM. The original version, MDIM 1.0, was released in 1991 and has been widely used despite increasing evidence of global, regional, and local positional errors compared to later, more accurate map products. The new mosaic, MDIM 2.0, has been made from the same set of ~4600 Viking Orbiter images as the original, but with improved photometric/cosmetic processing and substantially improved geodetic control. The root-mean-square (RMS) positional error between MDIM 2.0 and MOLA data is about 3 km. MDIM 2.0 is an interim product that will be superseded by later mosaics as described below. We have therefore mailed a limited number of copies to the NASA Regional Planetary Image Facilities (RPIFs) worldwide and to Mars mission teams. The compact disk (CD) volumes distributed are (as nearly as possible) identical to the 1991 design. The mosaic is also available through the Planetary Data System (PDS) Map-a-Planet website ( http://pdsmaps.wr.usgs.gov/maps.html), where users can order a custom map of any region of Mars and choose the resolution, map projection, and data format they require. Whereas retaining the 1991 archive design was an appropriate way to speed the release of the interim MDIM 2.0 product, we would like to improve upon this design for the three major global mosaic products to be produced in the next few years. These products are: 1) A final version of the Viking global mosaic (MDIM 2.1) with further improved cosmetic appearance and, more importantly, absolute positional accuracy on the order of the 231-m pixel size obtained by using MOLA data to constrain the control network. This mosaic is in production and should be finished late in 2001. It (and the products described below) will be mass-replicated and distributed to individual NASA planetary geology and geophysics investigators and to the RPIFs. 2) Updated versions of the Viking global color mosaic, which was produced in the early 1990s and was widely used but never officially released. Funding for these mosaics has been requested; if the project were approved, the mosaics would be completed late in 2002. The color images have resolutions close to 1 km/pixel; the mosaic would be produced at 1/64 degree (~926 m) per pixel, and a version merged with MDIM 2.1 at 1/256 degree (231 m) would be generated. 3) Color and monochrome mosaics based on MGS MOC images from the geodesy campaign. Red-camera images are available at 240 m/pixel (nadir) and blue at 480 m/pixel. From these a global quasi-true-color mosaic with a synthetic green channel will be produced at 1/256 degree resolution. In addition, nadir and oblique images from the red camera will be used to produce a stereopair of mosaics at this resolution, filtered and normalized to emphasize topographic features. We refer to these mosaics collectively as MDIM 3.0. Funding for their production has been allocated, with a target date for completion late in 2003. In addition to these major digital datasets, we also have proposed to make printed maps of Mars based on MOLA data. A global map on one sheet at 1:25,000,000 scale has been proposed for production in the coming year, and additional maps at larger scales would certainly be possible given the quality of the data. Although most of the issues we raise here concerning digital archive design are not relevant to printed maps, the choice of latitude-longitude coordinate system does affect printed as well as digital products. The desirability is obvious of making the design of the three new digital products as similar as possible. Technical advances since the 1991 release of MDIM 1.0 make some departures from the design of that product possible, and experience with that product and others over the past decade indicates additional changes that would be desirable. We summarize our best ideas for the design of future Mars digital maps below, contrasting them with the design of MDIM 1.0-2.0, and invite your comments and suggestions. Please respond no later than 13 July 2001 if you have objections to any aspect of the proposed design or wish to suggest modifications. DISTRIBUTION MEDIUM We will continue to archive and distribute digital cartographic products on optical disk media and to design the disks to meet the requirements of the PDS. Whereas MDIM 1.0 and 2.0 were released on CDs, we intend to use Digital Versatile Disk (DVD) media for future products. DVDs offer advantages in reduced cost (and bulk); until recently these advantages have been offset by the limited availability of DVD readers, but we believe that this is no longer a significant obstacle. The cost savings for distributing large datasets on DVD are substantial. A single-side, single-layer DVD holds 4.7 GB of data compared to 0.66 GB for a CD. This means that a single-band, 1/256 degree per pixel MDIM can be archived on a single such DVD rather than on six CDs. The cost of replicating a DVD is greater than that for a single CD, but the net cost of creating and shipping 200 copies of a dataset on DVDs is less than one-quarter of the cost of using CDs. DIGITAL SCALES We will continue to use scales (in degrees per pixel) equal to an inverse power of two as recommended by the NASA Planetary Cartography & Geologic Mapping Working Group (PCGMWG). This convention helps ensure that different datasets can readily be combined or compared by doubling or halving pixels to achieve a common scale. The Viking-derived MDIM 2.1 and MGS-derived MDIM 3.0 will both be produced at 1/256 degree/pixel scale. The Viking color mosaics, if produced, will be made at 1/64 degree and merged with MDIM 2.1 at 1/256 degree. TILING SCHEME The term "tiling scheme" refers to the division of the global mosaic into individual files. We strongly support the idea, also developed by the PCGMWG, that digital map products should be divided into files that correspond directly to a series of printed maps of the same body. The question remains as to which Mars map series is an appropriate basis for tiling future mosaics. MDIM 1.0 was divided into almost 2000 files corresponding to the quadrangles of the 1:500,000-scale map series. The small size (1.6 MB) of these files was appropriate in 1991 but modern computers can easily open much larger images and the need to piece together many small images is a source of frustration. We propose to base the tiling of future Mars mosaics on the 1:2,000,000-scale map series. This series contains 140 quadrangles and yields file sizes (for a single-band, 8-bit dataproduct at 1/256 degree per pixel) of about 22 MB. We believe that the 1:2,000,000-scale series yields the best compromise between file size and number of files. No published map series has a scale between 1:500,000 and 1:2,000,000, so if smaller files were desired it would be necessary to subdivide the 140 quadrangles into quarters of ~5 MB each. In the other direction, the 1:5,000,000-scale series has only 30 quadrangles and the largest of these are 160 MB in size, which is probably unacceptably large. Files will not be trimmed to the exact boundaries of the map sheet, but will include a border of extra pixels to ensure that no gaps form when adjacent files are mosaicked together. The number of pixels of extra data will be chosen to ensure that gaps do not arise even when files are rescaled by a factor of a power of two for comparison with other datasets. MAP PROJECTION The two projections appropriate for global digital databases are Sinusoidal and Simple Cylindrical. Of these, Sinusoidal projection has been used for past digital maps of Mars and other bodies, including MDIM 1.0 and 2.0. The equal-area property of this projection is useful in some scientific investigations and leads to a global archive that uses disk space relatively efficiently (at least when there are a large number of files; when the number of files covering the planet is small, the efficiency of Sinusoidal projection approaches that of Simple Cylindrical). The main disadvantages are that individual datafiles must be resampled before being mosaicked together because their center longitudes of projection differ and that for large regions the projection leads to objectionable distortions away from the equator and center longitude. Because the sides of the quadrangles are curvilinear in Sinusoidal projection, each file must be padded with a few pixels of data from adjacent quadrangles to prevent gaps from forming when files are reprojected and mosaicked. If the data are to be used at reduced scale then even more padding will be required. The use of Sinusoidal projection also complicates construction of the mosaic; either each quadrangle must be built as a separate mosaic, with the same images being map projected several times into neigboring quads, or mosaics of larger regions must be built and then resampled to the center longitudes of the multiple quadrangles they cover. To date the latter approach has been used despite the undesirable degradation of the image that results from the extra resampling step. Like Sinusoidal projection, Simple Cylindrical projection has distortions: both area and east-west scale are increasingly misrepresented toward the poles. . The main advantages of this projection are that any number of adjacent files can be joined without reprojection and that many software systems (e.g., for visualization of map data as virtual globes) understand Simple Cylindrical even if they handle no other map projections. The only parameters needed to describe a quadrangle in this projection are the digital scale and the latitude-longitude ranges. Another advantage is that, because quadrangle b oundaries are straight lines in the row and column directions, no padding of the files with data from adjacent quadrangles is needed to insure seamless mosaics. Users with access to software (e.g., ISIS, VICAR) that understands a variety of projections can produce maps with equal-area, conformal, equidistant, or other properties as desired from either Sinusoidal or Simple Cylindrical databases. Because we will, in the future, produce either type of database with a single resampling of the images, the quality of the results will be at least as good as past Sinusoidal maps. Unfortunately, not all users of the existing Mars data have access to such software, and, as a result, we routinely receive requests for digital map data in Simple Cylindrical projection. We therefore propose to make and archive all future global digital maps in Simple Cylindrical projection. The modest increase in required storage space is not a substantial deterrent, given modern storage media. Given that the proposed switch to Cylindrical projection is partly intended to meet the needs of users outside the NASA-funded community, we will be especially interested to hear whether there is a strong preference by members of that community (to whom ISIS and VICAR are available) for retaining Sinusoidal projection. If so, one possible compromise would be to produce PDS-formatted images in Sinusoidal and another format such as GeoTIFF (see next section) in Simple Cylindrical projection. DATA FILE FORMAT MDIMs 1.0 and 2.0, as well as numerous other digital map archives, have contained data files in formats approved by the PDS and unique to the NASA planetary cartography community. Such files include labels, readable both by humans and by machines, that provide a complete and well-documented description of the nature and map projection of the data in the file. The detailed information in the PDS labels maximizes the utility of data in this format for users with access to sophisticated planetary cartography software such as ISIS and VICAR. Multispectral data can be stored as multiband image "cubes" or as individual bands in separate files, and in either case the software mentioned provides freedom in comparing and displaying different spectral channels. The PDS standards also recommend that the image data be uncompressed; if desired, an entire image file can be archived in a standard losslessly-compressed (i.e., zip) format. This recommendation addresses the need to preserve the quantitative accuracy of the data for scientific analysis. We strongly agree with the rationale behind the PDS data format as just described and intend to always provide digital map archives in such a format. These PDS files would be provided in Sinusoidal projection for the reasons outlined above. There are uses, however, for which other formats may be more appropriate. Many users simply wish to view an image of part of Mars or to use it to prepare an illustration. Even if they have access to sophisticated cartographic software, which they may well not, translating PDS files into such a system, reprojecting, mosaicking, and combining spectral bands is a time-consuming process that must often be followed by translating the data into a more widely recognized format so it can be shared. We therefore propose to include with future archives data in widely readable non-PDS as well as PDS formatted files. The non-PDS files will have the same scales and extents. As discussed above, these files will be in Simple Cylindrical projection to facilitate combining them into larger regions, regardless of whether Simple Cylindrical or Sinusoidal projection is adopted for the PDS files. Where appropriate, three-band color data will be presented in a single file. We propose to adopt the GeoTIFF standard for the non-PDS image files. This format incorporates map-projection information into the labels of the well-documented TIFF format. Presently, only the map scale and a tiepoint giving the latitude and longitude of the first pixel are defined for bodies other than Earth, but this is sufficient to define a dataset in Simple Cylindrical projection. Some Geographical Information System (GIS) software (e.g., ArcView from ESRI) can read this projection information, and a much larger number of software packages will be able to open the images without comprehending the projection. The TIFF standard also includes a variety of image compression algorithms. Use of a lossy compression scheme can reduce the volume of a full-color image to substantially less than the uncompressed size of a single band without introducing visible artifacts. (If the unmodified data are needed they will be available in the corresponding PDS-formatted file). The TIFF standard also includes data types with more than one bit per pixel, useful for digital elevation models (DEMs) and other geophysical datasets, though not all software recognizes all TIFF pixel types. We will adopt a different non-PDS file format (e.g., JPEG) if users strongly prefer it, but the combination of basic map-projection information and flexible image compression makes TIFF the best choice in our opinion. EXTRAS The GeoTIFF files just described are one form of "extra" data that we propose to include in addition to the basic, PDS-formatted files. Past digital map releases have incorporated several other types of "extras," some of which we propose to include in the future as well. We welcome suggestions for extra datasets that would be useful without requiring large amounts of processing and/or storage space. Past PDS archives have included reduced-resolution versions of the data, and we plan to continue this tradition. For example, MDIM files are provided at 1/256 degree/pixel scale as described above, but files covering the same areas at 1/64 and 1/16 degree/pixel have been included in previous releases. These smaller files may be adequate for some scientific uses; in addition, they provide a means to make a quick, visual verification of the features covered by a given file. We will also continue to provide data near the poles in Polar Stereographic projection as well as in Simple Cylindrical (and possibly Sinusoidal) projection. MDIM 2.0 includes a set of text files containing detached ISIS labels for all the PDS images on a given disk. These files make it possible to open the images in ISIS without running the pds2isis translation program. The labels on the CD can be opened as if they were ISIS files, or an ISIS label and corresponding PDS image can first be copied to a hard disk directory and then be used. We plan to provide detached ISIS labels with future datasets. We unfortunately do not have the capability to provide VICAR labels as well. One "extra" provided as part of MDIM 1.0 (and duplicated with MDIM 2.0) is a set of files corresponding to the image mosaic files but containing scanned airbrush shaded relief map data. Because the original airbrush map has already been presented and there is no plan to adjust it to conform to the modern geodetic control network, airbrush data will not be included with future datasets. If you have a strong desire to see the airbrush map updated to conform to modern control, please let us know so we can propose to do this work. Similarly, the global digital topographic model on the seventh CD volume of MDIM 1.0 has been superseded by MOLA data and will not be included in future archives using the new format. (It is possible, however, that we will include the MOLA gridded elevation data in a format precisely matching future MDIMs as to scale, projection, and division of the planet, in order to simplify the use of the image and topographic data together.) COORDINATE SYSTEM We have left for last the most technically complex and potentially most scientifically complex issue. Two competing coordinate systems exist for Mars. In 1970, Commission 16 of the International Astronomical Union (IAU) defined these systems and approved their use. One combines longitude measured positive east with latititude measured from the equatorial plane to a point through the center of the planet, so-called planetocentric (areocentric for Mars, geocentric for Earth) latitude. This is simply a right-handed spherical-polar coordinate system. The other system uses longitude measured in such a direction that the sub-Earth longitude increases with time; for Mars, this means positive west. The second system also uses planetographic latitude, which is measured as an angle between the local vertical at a point and the equatorial plane. Because the shape of Mars is flattened relative to a sphere, the planetographic latitude of any point is greater in magnitude than the corresponding planetocentric latitude (except at the equator and poles, where the two types of latitude are equal). The maximum difference between the two types of latitude on Mars is about 0.3 degree or 20 km, at 45 degrees north and south. This is small relative to the possible difference of as much as 360 degrees between east and west longitudes of a point, but not negligible. Our present dilemma arose because each of the two different IAU-approved coordinate systems was adopted by one or more map producers. Mars maps (paper and digital) produced by the USGS since the 1970s use the west/planetographic system. Hundreds of maps have been made in this system and thousands or tens of thousands of literature citations identify features by their west/'ographic coordinates. More recently, the MGS MOLA and TES teams adopted the east/planetocentric system for their dataproducts. They have been followed by the Mars Odyssey Orbiter 2001 mission. We are proposing to adopt this east/'ocentric system for the future digital mosaics of Mars described above, and, indeed, for all future Mars maps. Other groups we have contacted, such as the NASA Geological Mapping Subcommittee (GEMS) and the ESA Mars Express High Resolution Stereo Camera (HRSC) team have expressed a willingness to follow the lead of the USGS and adopt our choice of coordinate system for their future products. We believe the balance of advantages lie with the east/'ocentric system and will adopt it unless we receive an overwhelming response to this email indicating that a majority of the Mars research community favors retaining the west/'ographic system. There are several points to be raised that may clarify our proposal to switch to the east/'ocentric coordinate system. The first, which is clear enough to need no further comment, is that the only reasonable time to make such a decision is now. Once we have started to produce the positionally accurate map products described above there will be an overwhelming incentive to use compatible latitudes and longitudes in all our future products. The second point is that the planetary research community cannot escape dealing with two competing coordinate systems for Mars. In this respect the situation is very different than before the MGS mission when only the west/'ographic system was in use in the literature and opposition to the use of east longitudes was vehement (though ultimately ineffectual). If the USGS retains the west/'ographic system for future maps, these will not be compatible with products generated by the MOLA, TES, and Odyssey teams. If future USGS maps use the east/'ocentric system for compatibility with these teams, they will be incompatible with the bulk of old map products from the USGS and elsewhere. We strongly believe that the latter situation is preferable because the accuracy of geodetic control networks of Mars has increased greatly since the older products were made. This means that new and old USGS map products would not align precisely even if they were produced in the same coordinate system. New maps made in the east/'ocentric system will agree closely with MOLA data, since their geodetic control will be tied directly to MOLA. As the new products enter service, the older, incompatible ones will naturally fade from use. This problem of compatibility is especially visible for the largest scale map series (the 1:500,000 geologic and topographic maps) where some but not all quadrangles have been mapped. If the new maps are made with boundaries at multiples of 5 degrees planetocentric rather than planetographic latitude, they will mismatch adjacent quadrangles in the old system by as much as 4 cm (scale equivalent of 0.3 deg or 20 km). This seems a serious objection, but in fact the geodetic errors in the older maps will result in mismatches of several centimeters even if the coordinate system is the same. This is a known hazard of making a map series while simultaneously refining the control network on which it is based. A third point concerns the relative technical merits of the two systems. This has aroused some strong statements in the past. We believe that the margin of advantage here is slender but goes to the east/'ocentric system. This system is "primary" in the sense that space remote sensing data are produced in it (or actually in the Cartesian coordinate system equivalent to it). Navigation of orbiters and landers, which is an important use for highly precise coordinate data, is also performed in this system. In practice, planetographic latitudes are derived by computation from the planetocentric angles determined by remote sensing. (Planetographic latitudes could be measured directly by a human surveyor with a theodolite on Mars, but this possibility seems too distant to carry much weight.) The most important result of planetographic latitudes being computed from primary, planetocentric measurements is that the transformation depends on the polar flattening of Mars. This is a measured quantity and our best estimate of it has changed significantly from Viking to MGS and even during the MGS mission. Because of this, different planetographic latitudes could be computed for the same feature at different times, quite apart from changes in the geodetic control net. These differences could be 1-2 km, enough to interfere with future precision landings if confusion about the correct planetographic coordinate system arose. The conversion from planetocentric to planetographic latitude also formally depends on the elevation of the point, but to date the elevation has been ignored in such calculations. This effect is <100 m even at the extremes of martian elevations, so the concern is more theoretical than practical at the moment. Another technical consideration is the compatibility of software with the two types of coordinate systems. The west/'ographic system has been standard for products generated in ISIS and VICAR, so these systems will need to be modified. As noted, both of them calculate with east/'ocentric coordinates and convert to the other system on output, so the required modification is relatively trivial. The latest release of ISIS already allows user control of the desired output latitude-longitude system. Third-party GIS and photogrammetric software systems pose more of a problem, in that their code is not accessible for modification. This does not favor either of the IAU-approved coordinate systems, however, since the most important such systems (ESRI ArcView, LH Systems SOCET SET) use a mixed east/planetographic system not approved by the IAU. Thus, one must convert data for use with these packages in either case. Our fourth and final point concerns the measures that we plan to take to minimize the confusion caused by a change to the east/'ocentric coordinate system. The most important is to document clearly and unambiguously the system used in each product. This means not only noting the direction of longitude and type of latitude (as well as the polar flattening used to compute planetographic latitude) in the labels of each digital map file and in the collar text of each printed map, but also including the direction of longitude in the name of the map. Thus, a quadrangle formerly known as 15N022 (centered at approximately 15 degrees north, 22 degrees west) would become 15N338E. If it were necessary to refer to the old designation, this would be clarified as 15N022W. Quadrangle designations in the 1:5,000,000 and 1:2,000,000-scale series, which are based on sequential numbering and feature names (e.g., MC-15: Elysium), would of course not be affected. On printed maps we also plan to aid users in locating features whose coordinates are given in the old system by supplying a secondary grid. The map boundary and gridlines would correspond to whole degrees in east/'ocentric coordinates but tickmarks and labels would be provided, probably in color, for west/'ographic coordinates. This approach was elegantly demonstrated by the DLR in simulated products for the Mars 96 mission (though at that time the primary coordinate grid was chosen to be east/'ographic). For digital products, the latest ISIS release already provides the capability to resample map files and convert between the two types of latitude and longitude. We thank you for your attention in reading this message and welcome your feedback on any aspects of the proposed dataproduct design. Please respond directly to Randy Kirk (firstname.lastname@example.org) no later than 13 July 2001. We hope you share our enthusiasm about the many high-quality Mars map products that will become available in the next few years. __________________________________________________ Dr. Randolph L. Kirk Astrogeology Team Ph. 520-556-7020 U.S. Geological Survey Fax 520-556-7100 2255 N. Gemini Dr. Flagstaff, AZ 86001 USA NOTE new email address: ASTROG::RKIRK and email@example.com are now firstname.lastname@example.org __________________________________________________