These tests lead to the conclusion that, given an adequate track type 

 survey design based on the frequency characteristics of the data, the cubic 

 spline algorithm is a reliable interpolation technique. The computational 

 efficiency of the algorithm is demonstrated by the fact that the 81 x 81 grid 

 values interpolated from the model-1 survey required only one minute and ten 

 seconds of computer time on a CDC-3800. 



As a qualitative test of the cubic spline gridding procedure applied to 

 actual shipboard survey data, the algorithm was used to grid total magnetic 

 intensity data obtained from a we 1 1 -control led marine magnetic survey consisting 

 of E-W tracks spaced nominally 3 nm apart. Figure 17 is the hand-drawn 

 contour chart of this area and Figure 18 is the machine contoured result using 

 the cubic spline generated grid points. Aside from the fact that the cubic 

 spline generated what is considered to be a more realistic position for the peaks 

 of the anomalies, the two contour charts are nearly identical. The computer 

 time required to generate the gridded data and plotter tape was less than three 

 minutes on a CDC 3800 with an actual plotter time of twenty minutes. This is 

 compared to an estimate of approximately 100 man-hours to plot the data values 

 and produce the hand-drawn contour chart. 



B. Mean Gravity Anomaly Calculations 



In order to test the algorithms for computing mean gravity anomalies, a 

 survey was designed which simulated a conventional shipboard gravity survey. 

 This survey plan, designated model 4, consists of E-W tracks spaced 3 nm apart 

 with a sampling interval of 1/3 nm. Figure 19 is a plot of this survey showing 

 the random deviation from the planned track which was added to simulate a 

 normal survey track. Figure 20 is a histogram of the errors in the interpolated 

 data points resulting from the model -4 survey. Comparing this result with that 

 obtained from the model-3 survey (Figure 14) indicates that the increased sampling 

 rate (six times that of model 3) was sufficient to overcome the effect of increasing 



36 



