Mars MOLA Mission Experiment Gridded Data Records (MEGDRs) are a relatively easy way to access extraterrestrial terrain data and one of the foundations for creating terrain maps for this interesting planet. As many know from creating terrain renderings on earth, three things are required: digital elevation data; a georeferenced image to drape, and the software to assemble and view the two. MOLA data is fundamental because it provides data for the entire planet, and because the useful 3DEM application can import the data directly.

MEGDRs have been produced at resolutions of 4, 16, 32, 64, and 128 pixels per degree. (A separate polar map data set is available in resolutions of 128, 256, and 512 pixels per degree.) You will need the highest resolution data available, which is the 128 pixel (or 512 pixel in the case of the polar data). This is not particularly high resolution. Data for the entire planet is contained in sixteen 125MB equatorial plus 4 512MB polar files. This is a lot of data but not compared to SRTM 90m earth data for example. The final version of the MEGDRs was released May 7, 2003.

The 128 pixel per degree data set available from NASA The ftp download procedure is as follows:


1. Create a new directory on your hard drive, for example "Mars MOLA 128.

2. Open your ftp client. I use the reliable (and free) Ipswitch products but there are dozens to choose from. Enter pdsgeosciences.wustl.edu/missions/mgs/megdr into the ftp address box. You can give the account any convenient name. It is best to explicitly specify binary file transfer mode, although auto detect may also work.

3. Use "anonymous" as the password and your email address as the user name.

4. Navigate to the /geo/mgs-m-mola-5-megdr-l3-v1/mgsl_300x/meg128 directory.

5. Select all of the 'megt… files and transfer them to your local directory.

In order to view a terrain rendering, consult the 3DEM user manual. I will summarize the gist of the procedure here without just repeating what you can read in the manual.

To use MEGDR files with 3DEM, choose 'File' | 'Load Terrain Model' from the 'Overhead View' menu to bring up the 'DEM File Type' dialog box. Choose 'Mars MOLA' as the DEM file type, and you will be presented with a 'Mars Area Selection' dialog box. Choose the 'MEG 128' Topography Data Set and then select a geographic location by clicking and dragging the white rubber band box on the map of Mars. Then click 'OK'.

3DEM will determine which MEGDR files are required and will present you with a file selection dialog box for locating these files. The first file to be located will be specified in the title bar of the file selection dialog box. The title bar will read something like 'Locate MOLA MEGDR File megt44n180hb.img'. Select this named file and click 'Open'. 3DEM will read and merge all of the files needed to produce a digital elevation model and draw an Overhead View centered on the coordinates you have chosen. Usually two or three MEGDR files are required, and as long as they are located in the same file directory you will only have to locate the initial requested file.

When you select 'Load Terrain Model' and 'Mars MOLA' from the 'File' menu you will be presented with a dialog box with a Martial terrain map. This map has nothing to do directly with your stored data and in fact will display whether you have downloaded the data or not. When you use the rubber band box to select an area of interest, 3DEM extracts the corner coordinates of the box. It then uses the database of stored MEGDR files to synthesize a display file. That is why it is important to download the entire MEGDR data set for the specified resolution. If a needed file is not in your local directory, 3DEM cannot construct the display file. Each file covers an area of 90 degrees width in longitude and 44 degrees height in latitude. The filenames specify the latitude and longitude of the NW corner of the included area. 3DEM provides a choice of either "Lat/Long" or "Sinusoidal" map projection when loading MOLA files.

The Lat/Long projection produces a rectangular surface grid with equal extension in latitude and longitude. The disadvantage of the Lat/Long projection is distortion at high latitudes. If you are creating images at high latitudes, try the Sinusoidal projection, which produces a surface grid corrected for latitude and a more realistic Overhead View and 3D projection.

When the display file is constructed, it will be rendered like any other DEM file of terrestrial data. You can also drape image data over the terrain data as for corresponding earth data files as explained in other articles in this website.

MOLA MEGDR data is also available for the north and south poles of Mars at the same ftp site cited above in the POLAR directory. Three data sets consisting of 128 pixels per degree, 256 pixels per degree, and 512 pixels per degree are available for each pole, requiring 6 image files (*.img), each again beginning with the letters "megt." As previously, I recommend selecting the highest resolution image set i.e. 512 pixels per degree. Each file is about 200 megabytes. Download all of the files of your selected resolution into a single directory on your hard drive. As before, 3DEM will automatically merge the needed files as necessary to create a seamless digital elevation model.

To use MOLA POLAR files, choose 'File' | 'Load Terrain Model' from the Overhead View menu to bring up the 'DEM File Type' dialog box. Choose 'Mars Polar MOLA' as the DEM file type, and you will be presented with a "Mars Area Selection" dialog box as shown below. Choose the 512 pixel/degree Topography Data Set, and select either the 'North Pole' or 'South Pole' data set. Now select a geographic location by clicking and dragging the white outline rectangle on the map of Mars. The green boundary rectangle indicates the geographic limits of data in the selected POLAR file. As previously, you will be prompted with a dialog box with a file name at the heading. Select the indicated file name and 3DEM will do the rest of the merging and rendering work.

The examples to the right show some of the images that I was able to create using the technique outlined above. Hopefully this will provide a helpful way to start exploring this most interesting of planets in our solar system.