A Burroughs 220 computer is being used at Stanford for fitting contour maps to facies data. The computer fits a smoothly curving parabolic surface to the data and then prints a contour map of the surface. Human error in fitting the contours is virtually eliminated. The method is particularly useful where direct contouring of data values is difficult or meaningless because of appreciable random fluctuations and error. The map printed by the computer smooths over the irregularities and shows the general trends. The contoured surface is fitted so that the sum of the squares of the numerical differences of the actual data above and below the surface is the least possible.

This method has been useful in mapping facies variations in a Pennsylvanian limestone bed in New Mexico and variations of modern unconsolidated carbonate sediments in the Gulf of Mexico and adjacent coastal swamps of the Everglades in southern Florida. Ratios, percentages, and average grain diameters mapped in these applications revealed systematic trends that were previously obscure.

Areas mapped by this method must be rectangles, but sampling localities, which may be measured stratigraphic sections, oil wells, or places where sediment samples were obtained, can be arranged irregularly within a rectangle. An arbitrary geographic coordinate system is established so that each locality is described by two coordinate values. The coordinate units may be feet, miles, or, as has been proved convenient, tenths of an inch scaled off the map. The coordinate values and the value to be contoured for each locality form the basic data fed to the computer.

The general instructions for the method are fed from paper tape into the computer's memory before the computation begins. In addition, the computer must be instructed as to the contour interval and the dimensions of the map that it is to print. The contours printed out by the computer consist of individual bands of a letter or symbol, such as A, B, and $. Each band spans one contour interval, and blank bands alternate with printed bands.

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