PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2017-08-25 NOTE = "The MGS MOLA Mission Experiment Gridded Data Record (MEGDR)" END_OBJECT = TEXT END The MGS MOLA Mission Experiment Gridded Data Record 1. Introduction This document provides an overview of the Mars Global Surveyor MOLA MEGDR Archive, including a complete listing of the contents of the data set. This data set contains map products derived from altimetry data observations acquired by the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) instrument. MOLA data products include Aggregated Experiment Data Records, or AEDRs (raw data), Precision Experiment Data Records, or PEDRs (data from AEDRs with precision orbit corrections applied), and Experiment Gridded Data Records, or EGDRs (gridded products derived from PEDRs). Two types of EGDRs are produced, Initial Experiment Gridded Data Record (IEGDR) and Mission Experiment Gridded Data Record (MEGDR). This archive contains MEGDR products, a result of accumulated altimetry observations over the course of the whole MGS mission. 2. Documentation The MEGDR data set is described in the file MEGDRDS.CAT in the CATALOG directory. Also in this directory are files describing the MOLA instrument (INST.CAT), the MGS spacecraft (INSTHOST.CAT) and mission (MISSION.CAT), personnel associated with this archive volume (PERSON.CAT), and a bibliography of references mentioned in these files (REF.CAT). The files in this directory are used to generate entries in the Planetary Data System online catalog. The DOCUMENT directory includes the Software Interface Specification (SIS) document for the MEGDR data set as a PDF file and as an HTML file. 3. Data Set Description The MEGDR products in this data set are images gridded at several resolutions: 4, 16, 32, 64, and 128 pixels per degree. These images provide maps of the global topography of Mars. In addition, maps with resolutions of 128, 256 and 512 pixels per degree are included for the north and south polar regions of Mars. For each map resolution, the data set contains images of planetary radius, topography, and the number of MOLA shots within a grid cell (referred to as counts). Topography is computed as the planetary radius minus the areiod raduis. Images of the Mars areoid are provided at resolutions of 4, 16, and 32 pixels per degree. See the DS.CAT file in the CATALOG directory for details about the areoid model used for this data set. The global maps are in simple cylindrical map projections using the IAU2000 planetocentric coordinate system with east positive longitude. The polar maps are stored in polar stereographic projections. Information about the map projection is found in the PDS labels. Formulas for converting between latitude-longitude and line-sample coordinates are in the files DSMAP.CAT and DSMAP_POLAR.CAT in the CATALOG directory. Simple cylindrical maps with resolutions of 4, 16, and 32 pixels per degree are stored in a single file. Maps with higher resolutions are divided into tiles. The 64 pixel per degree maps are divided into 4 separate images, with each image covering 90 degrees of latitude and 180 degrees of longitude. The 128 pixel per degree maps cover the latitude range of 88 degrees north to 88 degrees south and are divided into 16 separate tiles with each tile containing 44 degrees of latitude and 90 degrees of longitude. Each polar stereographic map is stored in a single file. 4. File Formats Most images within this data set are composed of 16-bit MSB-first signed integer pixels. The counts images for the 64 and 128 pixel per degree resolution maps are stored as 8-bit unsigned integer pixels. Each image has an accompanying PDS label that describes the format and content of the image. Each PDS label is in a separate file with the same name as the image it describes but with the extension .LBL. All text files in this data set are stream format files, with a carriage return (ASCII 13) and a line feed character (ASCII 10) at the end of the record. This allows the files to be read by the MacOS, Windows, LINUX, and UNIX operating systems. System utilities are available on the various computer types to convert this format to the internal format if necessary. Macintosh - Apple File Exchange, MS-DOS to Mac option. Unix - Translate utility (tr -d'\15' output_file) - DOS2UNIX utility (dos2unix input_file output_file) The SIS document in the DOCUMENT directory is given as hypertext (HTML), which can be viewed with a Web browser, and in the proprietary Adobe Portable Document Format (PDF), which can be viewed with the free software Acrobat Reader from Adobe Systems Incorporated. 5. Image Display The PDS provides software for displaying PDS-labeled images on a variety of computer platforms. The application program for image display is called NASAView, which has versions for SUN/SOLARIS, WINDOWS, and LINUX platforms. The PDS distributes NASAView through its Web site. Consult the PDS Web site for the status of NASAView in terms of its capabilities and availability. The address for the PDS Web site is http://pds.jpl.nasa.gov/. ENVI and ArcGIS are two popular commercial software packages that may be used to display the MEGDR maps. They are not able to import the map projection information directly from the PDS labels, so ENVI header files are provided for this purpose in the EXTRAS directory of this archive. See EXTRINFO for more information. 6. Archive Contents Files within this data set are organized into a series of subdirectories below the top-level directory. Top-level Directory ------------------- AAREADME.TXT The file you are currently reading. ERRATA.TXT This file lists known errors and anomalies for this archive. It is occasionally updated if additional notes about the archive are needed. VOLDESC.CAT This text file contains a description of the volume contents as a PDS catalog object. It is a required file for PDS archives. CATALOG Directory ----------------- Files in the CATALOG directory are text files containing documentation formatted in PDS object description language. The files contain information about the MGS mission and spacecraft, the MOLA instrument, the MEGDR data set, personnel associated with the data and archive, and references. See the file CATINFO.TXT for details. DOCUMENT Directory ------------------ The DOCUMENT directory contains the data set SIS document in both HTML and Adobe Portable Document Format (PDF). See the file DOCINFO.TXT for additional details. INDEX Directory --------------- The INDEX directory contains PDS index files for this archive. An index file is an ASCII table with each record (or line) in the table containing information about a single image in the archive. See the file INDXINFO.TXT for additional details. Data Directories ---------------- The MEGDR data files are stored in directories based on the resolution of the maps. There is one directory for each map resolution within the data set. Data directory names for the global maps have the form MEGxxx, where the xxx refers to the map resolution in pixels per degree, including leading zeros. For example, the directory MEG016 contains all the 16 pixel per degree maps. The polar maps are stored in a directory named POLAR. File names for the global image maps use the format of MEGpxxnyyyrv.IMG with the following definitions: p indicates the product type (A for areoid, C for counts, R for radius, and T for topogray) xx is the latitude of the upper left corner of the image n indicates whether the latitude is north (N) or south (S) yyy is the east longitude of the upper left corner of the image r is the map resolution using the pattern c = 4 pixel per degree e = 16 pixel per degree f = 32 pixel per degree g = 64 pixel per degree h = 128 pixel per degree i = 256 pixel per degree j = 512 pixel per degree (This convention is consistent with that used for the Mars Digital Image Model [MDIM] archives.) v is a letter indicating the product version. The file naming scheme used for the polar maps is MEGT_p_rrr_v.IMG, in which p is replaced by N for north pole or S for south pole, rrr is replaced by 128, 256 or 512, and v is the product version. The detached PDS label files use the same naming convention with .LBL instead of the .IMG of the image file. EXTRAS Directory ---------------- The EXTRAS directory contains a set of header files for use with the commercial image processing software ENVI (http://www.harrisgeospatial.co.uk/ProductsServices/ENVIProducts.aspx). One ENVI header file is provided for each MEGDR image product. The header files have the same names and subdirectory paths as the image files, but they have the file name extension ".HDR". See the document EXTRINFO.TXT for more information. 7. Whom To Contact For Information For questions concerning this data set: PDS Geosciences Node Washington University Dept. of Earth and Planetary Sciences 1 Brookings Drive St. Louis, MO 63130 314-935-5493 WWW Site: http://wwwpds.wustl.edu Electronic mail address: geosci@wunder.wustl.edu MOLA Science Team Code 920 NASA / Goddard Space Flight Center Greenbelt, MD 20771 WWW Site: http://ltpwww.gsfc.nasa.gov/tharsis/mola.html Electronic mail address: Gregory.Neumann@GSFC.NASA.GOV 8. Cognizant Persons MOLA data were provided by David Smith, MOLA Principal Investigator, and Greg Neumann, both of NASA/Goddard Space Flight Center. This volume was designed and produced by Edward A. Guinness and Susan Slavney, Planetary Data System Geosciences Node, Washington University, St. Louis, Missouri. 9. Citation The MOLA MEGDR data set may be cited in published literature using the following reference. Smith, D., G. Neumann, R. E. Arvidson, E. A. Guinness, and S. Slavney, "Mars Global Surveyor Laser Altimeter Mission Experiment Gridded Data Record", NASA Planetary Data System, MGS-M-MOLA-5-MEGDR-L3-V1.0, 2003.