Abstract: Method and apparatus are described for determining the contour lines for gridded data defining a four-sided cell composed of two grid lines along the X-axis and two grid lines along the Y-axis, involves the steps of sorting four data points according to their values along a selected one of the two grid axes; connecting the point of minimum value along the selected grid axis to the point of second minimum value along the selected grid axis; and connecting the point of third maximum value along the selected grid axis to the point of maximum value along the selected grid axis. In the described embodiment, the contour lines represent common elevation points in a topographic map.