Mars Global Surveyor: MOLA MEGDRs

August 25, 2017. ENVI header files for all MEGDR images are provided in the EXTRAS directory. These files are provided because ENVI is not able to import the map projection information directly from the MEGDR labels. See EXTRINFO.TXT for more information.

Mission Experiment Gridded Data Records

The MOLA Mission Experiment Gridded Data Records (MEGDRs) are global topographic maps of Mars created by binning altimetry values from the MOLA PEDR products acquired over the entire MGS mission. MEGDRs have been produced at resolutions of 4, 16, 32, 64, and 128 pixels per degree. (Polar maps are in resolutions of 128, 256, and 512 pixels per degree.)

The final version of the MEGDRs was released May 7, 2003. These products supersede all earlier releases of MEGDR and IEGDR  (Initial Experiment Gridded Data Record) products. A MEGDR file may be identified as a final version by these keywords in its PDS label:  PRODUCT_VERSION_ID = "2.0" and PRODUCT_CREATION_TIME = 2003-03-21 or later.

A MEGDR product consists of a set of three or four maps:

1. The planetary radius as recorded by the MOLA instrument

2. The areoid, a model for an equipotential surface of Mars, analogous to "sea level" on Earth (this map is omitted for the 32, 64 and 128 pixel/degree and polar MEGDRs)

3. The topography computed as the difference between the planetary radius and the areoid

4. Counts (number of observations) per cell in the map

MEGDR Data Online

Use the links below for direct access to any part of the MOLA MEGDR archive.

Online Tools

Mars Orbital Data Explorer - Provides search, display, and download  tools for this data set.

MEGDR File Naming and Organization (for polar files, skip to here)

MEGDR data files are named according to the scheme MEGkxxdyyyrv.IMG, where:

k = A for areoid, C for counts, R for radius, T for topography

xx = latitude of pixel in upper left corner of the image

d = N for north latitude, S for south

yyy = longitude of the pixel in the upper left corner of the image (0-360 east)

r = map resolution in pixels per degree, e.g.

C = 4 pix/deg

E = 16 pix/deg

F = 32 pix/deg

G = 64 pix/deg

H = 128 pix/deg

v = version letter.

The 4, 16, and 32 pixel/degree MEGDR maps are stored as images in single files, one file for each of the planetary radius, areoid, topography, and counts maps. Each file is accompanied by a PDS label in a separate file with the same name, extension ".LBL". The label is a text file that describes the content and format of the map.

MEGDR Image Files and Labels at 4, 16, and 32 Pixels Per Degree

The 64 and 128 pixel/degree MEGDR maps are so large that they are broken into smaller files called tiles for easier handling. Each file is accompanied by a PDS label.

MEGDR Tiling Schemes at 64 and 128 Pixels Per Degree

MEGDR Tiled Image Files and Labels at 64 Pixels Per Degree

MEGDR Tiled Image Files and Labels at 128 Pixels Per Degree

North and South Polar Map File Names

North and south polar maps have names in the form MEGk_p_rrr_v.IMG, where

k = "T" for topography, "R" for radius, or "C" for counts
p = "N" for north pole or "S" for south pole
rrr = resolution in pixels per degree (128, 256, or 512)
v = product version.

Polar images are square, with the pole at the center, lines of latitude in concentric circles around the pole, and lines of longitude radiating from the pole.  In the north pole image, Longitude 0 extends straight down from the center and Longitude 90 East extends to the right. In the south pole image, Longitude 0 extends straight up from the center, and Longitude 90 East extends to the right. For map projection details, see the map projection information in the image labels and the file DSMAP_POLAR.CAT.

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