when the radar altimeter experiment has been refined to the point where 

 geoidal undulations to the accuracy of a few tens of centimeters can be determined, a 

 comparison with a gravimetrically determined geoid determined to the same accuracy 

 could be of great use to physical oceanographers. Differences between the two 

 geoids will be related primarily to currents and tides, etc. in the oceans. 



2. SEA GRAVITY DATA IN THE WESTERN NORTH ATLANTIC 



There are two principal problems in the utilization of existing sea gravity 

 data for the construction of a gravimetric geoid. One problem is that the accuracy 

 of older surface ship gravity data is poor; the second arises from the uneven 

 areal coverage of gravity measurements. We consider these two problems in some 

 detail below. 



Gravity values obtained from svibmarine pendulum measurements in the western 

 North Atlantic have been given by Vening Meinesz (1948) and Worzel (1965) . The 

 locations of the pendulum measurements are given in Figure 1. These values are 

 accurate to a few milligals but the density of measurements is insufficient for the 

 present study. 



The locations of surface ship gravity measurements are also shown in Figure 1. 

 These measurements were made in the period 1961-1971. The Lamont-Doherty measure- 

 ments made aboard research vessels VEMA and ROBERT D. CONRAD, as well as the Dutch 

 measurements aboard H. NETH. M. S. SNELLIUS, utilized the Graf As)cania sea 

 gravimeters. The Woods Hole measurements aboard research vessel CHAIN used the 

 LaCoste Romberg gimbal mounted gravity meter in earlier measurements, and the 

 vibrating string gravimeter in later measurements. 



The accuracy of gravity measurements has increased steadily since the beginning 

 of surface ship measurements. The earlier measurements seldom had an accuracy of 

 better than 5 mgal. Under poor sea conditions the errors were as large as 20 mgal . 

 The latest measurements almost always have an accuracy of better than 5 mgal. When 



23-3 



