This is a surface fitting procedure that utilizes elevation contours to create an interpolated grid surface. The standard form, implemented in TNTMips for example, is a simple linear interpolation based on the nearest pair of contour lines. Nearest is defined as the closest to the selected grid point in any one of 8 directions (i.e. four direction pairs). The resultant grid will provide an exact interpolation at contour locations but will be discontinuous across contour lines. This procedure will flatten ridges and “fill” valleys unless a non-linear interpolant is used. For terrain surfaces procedures that correctly model the hydrological structure of the landscape are preferable, e.g. see Section 6.6.14, Topogrid/Topo to raster).