The residual anomaly is defined as the deviation from the mean anomaly surface, or the regional surface. The regional surface which best fits the observed anomaly data may be determined by least squares. For the case of the simple plane, the equation of the regional would be Z = Ax + By + C, and the residual anomaly would be R = G-Z = G-(Ax + By + C), where G is the observed value at the station whose coordinates are x, y. The constants of the equation of the regional may be determined by using the normalizing equations Sigma Rdelta R/delta A = 0; Sigma Rdelta R/delta B = 0; Sigma Rdelta R/delta C = 0. It is shown that for a symmetrical distribution of observational points about a center, the regional value at the center would be equal to Sigma n1 G n /n and this value subtracted from the central point is the residual. Given the coordinates and the observed values, the regional may be determined, and the residual for the area calculated rapidly. An example is given of the procedure.

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