of the manner in which the errors change allow us to apply adjustments to 

 measured ship values. Thus, for instance, where a correction is determined at 

 one point along a track segment (from an intersecting track of high accuracy) , 

 the same correction can be applied for the entire track segment where course 

 remains unaltered. The earlier gravity measurements have been adjusted in this 

 manner. We feel that this adjustment has been extremely important in increasing 

 the overall accuracy of the determination of the gravity field. 



The correlation of the short wavelength gravity undulations with topography is 

 demonstrated in Figures 4 and 5. We see that this correlation is good not only in 

 an area of the Mid-Atlantic Ridge where in an area of small thickness of sediments 

 one would expect such a correlation, but, surprisingly, also in an area east of the 

 Lesser Antilles where there are large anomalies due to subbottom density contrasts. 

 The bottom topography in the North Atlantic is known in much greater detail through 

 many extensive surveys, than the gravity field is. The high degree of correlation 

 between short wavelength gravity and topographic variations allows us to interpolate 

 gravity values between observation points on the basis of topography. 



We have utilized knowledge of the systematic manner in which errors in gravity 

 measurements change, as well as the knowledge of correlation of gravity with topography 

 to construct contour maps of free-air gravity in the western North Atlantic south of 

 45°, the Caribbean Sea and the Gulf of Mexico (Rabinowitz and Talwani, 1969; Talwani 

 and Poppe, 1968) . These maps have a contour interval of 25 mgal but a large number 

 of spot measurements are given which help in the interpolation of values between 

 contours where the contours are widely spaced. Spot measurements are also given for 

 maxima and minima. 



The gravity contour maps were used to obtain free-air averages over 1x1° squares 

 in the following manner. In areas where the gravity variations are relatively small 

 and the distance between contours is large, each 1x1° square was divided into nine 

 smaller squares. The values at the mid point of the smaller squares were visually 



23-8 



