axis C = 9.929 km. With the control network, a global 
topographic map (1:100,000) and a larger-scale 
topographic map (1:25,000) of Phobos were compiled 
with contour intervals of 500 m and 20 m respectively. 
6.5 Mars Digital Terrain Model (DTM) 
Mars DTM was derived as a by-product of the Mars 
topographic mapping and was produced by digitizing 
contour maps of Mars. There are two versions of Mars 
DTMS, a moderate resolution from the 1:15,000,000- 
scale topographic maps, and a high resolution from 
1:2,000,000-scale topographic. maps. The Mars 
DTM's use Sinusoidal equal-area projection and have 
been transcribed to optical disk (CD-RAM Volume 7 - 
V02007) of the planetary data system. By using the 
Mars DTM, a color-coded global map of Mars 
topography was generated in both the Sinusoidal 
equal-area projection and the Mercator projection. 
6.6 Quantitative Analyses of Mars Topography 
By using contour maps and the DTM, we have made 
quantitative analyses of Mars topography that Mars 
elevations above and below the topographic datum 
are found to be 67% and 33% , respectively. The 
average elevation of the western hemisphere (0° - 
180°) is about 0.993 km higher than the eastern 
hemisphere (180° - 360°; also, the southern 
hemisphere averages about 3.191 km higher than the 
northern hemisphere (due to a shift of Mars center of 
mass by approximately 3.4 km to the north of the 
center of figure.) The mean elevation is about 1.876 
km above the datum. The volume of the western 
hemispheres about 72 million cubic km greater than 
the eastern, and the southern hemisphere is about 244 
million cubic km greater than the northern. The total 
global volume of Mars is about 163.2 billion cubic km. 
7. DISCUSSION 
Because Mars missions were not specifically planned 
for making three-dimensional photogrammetric 
measurements, these data do not fulfill mapping 
requirements and therefore attribute in poor precision 
of topographic mapping. For example, by using Viking 
Orbiter images of Mars, compilation is limited by 
factors such as: low-resolution photography, very 
narrow field-of-view of the camera, weak geometry 
(i.e., small base-to-height-ratios), the presence of haze 
and mist in the Martian atmosphere, and differences in 
the direction of illumination in pairs of stereo-images. 
In any rate, these topographic products of Mars are the 
first and will retain as the most comprehensive 
topographic maps until the next enhancement using 
updated topographic data from future Mars Missions. 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996