A method of emphasizing anomalies of a certain size in a potential-field map. A grid (usually square or triagonal) is drawn on a contour map and values are determined at the grid intersections by interpolation. The residual at one of the grid intersections is the value at that point less the average at other intersections a fixed distance away. Averages at several distances may be used and weighted to approximate second-derivative or other functions. The process of making grid residuals is also called map convolution (a 2D convolution) because it represents map data convolved with a residualizing operator (or template). This technique has largely been replaced now by gradient filters and other digital high-pass filters. See also residualize.