Specifying Contour Intervals
SURFACE III provides two mechanisms for specifying the way contour lines will be drawn on a map. Usually, contour lines are equally spaced to cover the range in values of the surface being mapped. The interval between such equally spaced contours is specified by the Contour Interval command, set from the main Contour dialog. However, contour lines at arbitrary levels can be specified with the Levels command, which reads a sequence of contour levels from an external file. Both commands have parameters which allow the user to specify the form of annotation on the contour lines. Hatchuring and other special forms of lines may be specified.
Surface III will automatically assign contour intervals if these are not specified. If the smallest map dimension is less than 10 inches or 25 centimeters, the increment between successive contour levels will be adjusted to a rational value that will yield a maximum of fifteen contour intervals covering the range of the original data. If the smallest map dimension is less than 20 inches or 50 centimeters, a maximum of thirty contour intervals will be generated. If the smallest map dimension exceeds 20 inches or 50 centimeters, up to sixty contour intervals will be used.