Values in Canada obtained by the Earth Physics branch of the Department of 

 Energy, Mines and Resources were taken from Walcott (1970). In Walcott's study, 

 free-air gravity contours were based on averages over 1x2° squares. The values in 

 Canada in Figure 6 have been obtained by interpolation from the contours. The values 

 in the United States were obtained principally from a compilation by Strange and 

 Woollard (1964) . 



4. METHOD USED IN CONSTRUCTION OF GEOID MAPS 



Basic Approach 



We have employed Stokes theorem for obtaining geoid heights. The free-air 

 anomalies averaged over 1x1° (and some 5x5° squares) as shown in Figure 6, were 

 utilized. Outside the area in which anomalies are given, we assume that the gravity 

 field is determined by the spherical harmonic coefficients for the geopotential to 

 order and degree 16 from the combination solution of Gaposchkin and Lambeck (1971) . 

 In other words, for the near zones we used the averaged free-air anomalies; for distant 

 areas we used gravity values from Gaposchkin and Lambeck 's combination solution. 



The actual construction of the geoid is as follows. Geoidal heights were 

 determined from Gaposchkin and Lambeck 's combination solution in the western 

 North Atlantic and over North America. The geoidal heights have been contoured at 

 5 m intervals and are shown in Figure 8. We term this geoid the "G and L" geoid for 

 the purpose of this paper. Similarly, the spherical harmonic coefficients were used 

 to compute gravity values averaged over 1x1° squares for the western North Atlantic. 

 These "G and L" gravity values were subtracted from the free-air gravity values 

 averaged over 1x1° squares (surface data shown in Fig. 6) . The gravity differences , 

 also expressed as 1x1° squares were then used to compute a "difference" geoid (Fig. 9) . 

 The "1x1° difference" geoid represents the additional information provided by the 

 1x1° surface gravity data. The "1x1° difference" geoid is then added to the "G and L" 

 geoid to obtain a "1x1°" geoid (Fig. 10). The "1x1°" geoid uses 1x1° surface gravity 

 data in the area of the maps, but gravity data from Gaposchkin and Lambeck ' s combination 

 solution outside the area of the maps. The practical advantage of the above procedure 



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