Mission-Tracking Data. By using Lunar Orbiter IV tracking data and laser 
ranging data, gravity coefficients of the lunar gravity field were developed by 
W.L. Sjogren (Ferrari, et al., 1980). Based on the newly derrived gravity 
field, a new topographic datum of the Moon has been derived in terms of 
spherical harmonics of fith-degree and fifth-order with the sixth-degree 
sectorial terms (Wu, 1981b). From the results of the celestial mechanics 
experiment of Mariner 9 (Jordan and Lorell, 1975), the topographic datum of 
Mars has also been derived based on the fourth-degree and fourth-order 
spherical harmonics of the Martian gravity field (Wu, 1981b). These two datums 
have been adopted to be the 0-elevation reference surface of all topographic 
maps of the Moon and Mars. 
  
Pioneer Venus Radar Altimeter Data. With a wavelength of 17cm, the radar 
altimeter aboard Pioneer Venus covered about 93%, between 74° North latitude 
and 63° south, of the Venusian surface. The resolution of the radar altimeter 
data is about 150km with an accuracy of about 200m (Masursky, et al., 1980). 
Using the Pioneer Venus altimeter data, a preliminary global topographic map of 
Venus has been compiled at a scale of 1:50,000,000 with a contour interval of 
lkm. As shown in Figure 2, Venus has a relief difference of 13km. The terrain 
of Venus consists of 8% highland (6053-6062km); 65% upland rolling plains 
(6051-6053km), and 27% lowland (6049-6051km) (Masursky et al., 1980). 
  
  
  
Fig. 2. PRELIMINARY TOPOGRAPHIC MAP OF VENUS--EQUATORIAL REGIION. (Map was 
compiled, at a scale of 1:50,000, using radar altimetry data of the 
Pioneer Venus mission. The contour interval is lkm. Elevations 
are referred to a mean radius of 6051.5km). 
28 
  
   
     
    
    
  
    
    
     
   
    
       
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   
   
   
  
  
  
    
     
  
    
    
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