with the astrogeodetic profile north of 35°N. South of 35°N the "1x1°" geoid 

 is too low. The "G and L" geoid in this region is too high. We believe that errors 

 in the gravity values used for Mexico are responsible for giving "1x1°" geoid 

 values that are somewhat too low. 



The agreement with the profile along 35°N is also good. The RMS difference 

 amounts to 1.3 m. If the noticeably large disagreement between 90°W and 95°W is 

 not considered the RMS difference drops to slightly less than 0.9 m. 



6. COMPARISON OF GRAVITY AND GEOID MAPS 

 We shall discuss the important features of the gravity map of Figure 7 and 

 then examine the geoid map to see which gravity highs and lows can be seen in the 

 geoid map. If, in the future, geoid maps obtained by satellite based radar 

 altimeters are to be used for geophysical purposes, it is of great interest to 

 examine the magnitude of the geoid undulations corresponding to important gravity 

 features. 



In comparing the gravity map (Fig. 7) with the geoid maps it would seem at first 

 sight most logical to make the comparison with the "1x1°" geoid map of Figure 10, 

 since both maps are referred to the same ellipsoid. However, the geoid map is 

 much more sensitive to the low order harmonics of the gravity field which account 

 for the large regional gradient in the geoid map. Therefore it is more useful to 

 compare the features in the gravity map with corresponding features in the "1x1° 

 difference" geoid map. The "1x1° difference" geoid, in effect, uses the "G and L" 

 geoid as the reference surface. It thus principally reflects short wavelength 

 gravity features which are also prominent in the gravity map of Figure 7. 



In comparing the gravity and geoid maps note that in areas where we have used 

 5x5° averages the data are generally poor. The contours in such areas are dotted. 



23-29 



