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III. TECHNIQUES OF PLANETARY TOPOGRAPHIC MAPPING 
  
The techniques applied with imaging data for planetary topographic mapping 
are different from the techniques applied with the nonimaging data. With the 
imaging data, some new equipment and methods were developed, and in some cases 
conventional photogrammetric methods and equipment were modified to perform map 
compilations. With the nonimaging data, contour lines are interpolated from 
discrete data points. 
With the nonimaging data,the procedure involves the computation of the 
elevations -of all of the data points with respect to the adopted topographic 
datum, the adjustment of all the reduced elevation points to be in agreement 
with certain control points, and the compilation of contour lines by 
interpolation using all of the elevations available. 
The true elevation of a point is obtained by comparing the observed radius 
of the topography with the computed radius (topographic datum), if the data 
point is the observed radial distance and has not been reduced to the datum. 
If elevations of data points have previously been reduced to an arbitrary 
datum, the elevations are then converted back to their radius value and 
compared with the radius of the new topographic datum. 
For the compilation of the global map of Mars (Figure 1), the radio- 
occultation points of Mariner 9 were used as control points. Since Mariner 9 
had an inclination angle of 65°, all of the data. from both the UVS and the IRIS 
experiment were somewhat oriented in the north-south direction and the paths of 
the Earth-based radar data are exactly in the east-west direction in two belts 
in both the northern and southern hemispheres, the adjustment procedure is 
therefore to first adjust the radar data by matching them to the occultation 
points, then all the data of the UVS and IRIS elevations were adjusted by 
matching them to the adjusted radar points as well as the occultation points at 
their mutual intersections. This was done like laying down railroad ties (data 
on paths) onto two railroad tracks (radar data belts). Contour lines were then 
drawn by interpolation using all of the adjusted elevations. Figure 1 is the 
map compiled by this technique. This map is currently being updated with the 
large amounts of imaging data from the Viking mission and with additional radar 
observations. 
In order to use the Viking Orbiter photographs, which have extremely 
narrow fields of view, for systematically mapping Mars, special techniques have 
been developed (Wu, et al., 1982). 
Using topographic information derived from lunar photographs, laser 
altimetry, lunar radar sounder, and Earth-based observations, a new global 
topographic map of the Moon is under compilation at a scale of 1:5,000,000 with 
a contour interval of 500 meters. An intermediate product of the compilation 
is contour maps compiled by photogrammetric methods, using metric photographs 
of the Apollo 15, 16, and 17 missions (Wu, 1981a). Maps were compiled using 
the same format as the existing NASA Lunar Planning Charts and therefore the 
map scale is 1:2,750,000 with a contour interval of 500 meters. Figure 3 is an 
example of the eastern half of the map LOC-3 which covers an area between the 
longitudes 140°E to 40°W. 
IV. DISCUSSIONS 
The data used for planetary topographic mapping is entirely generated from 
remote sensing. Map precision depends upon the sensitivity of the sensor 
devices and the methods used to interpret the remote sensing data. New mapping 
techniques and the modification of existing mapping techniques are continually 
in progress. For instance for the mapping of Venus in great detail, 
topographic mapping using side looking radar images is under development.