Such models are useful when only a single view is available, for example, planning military missions, imaging rapidly-changing environmental situations, or mapping other planets with spacecraft imagery. One common means of producing three- dimensional views from single images is to use the relationship between light reflectance and angle of illumination to estimate the slope of a surface. The slope information can then be used to calculate altitude. If the slope information can be controlled by elevations of known points, the accuracy of the model is greatly improved. Topographic maps display elevation information by means of contour lines, lines of equal elevation.
Contour lines convey much more information than simply elevation.
For example, the relative spacing of contour lines shows the steepness of the terrain. Widely-spaced contour lines indicate gentle slopes, that is, the terrain rises or falls only a small amount in a great distance. The texture of contour lines is also quite revealing. An area with gently-curving contours has relatively smooth topography, whereas intricate, jagged contours suggest rough terrain. Crossing such terrain might be difficult, even if the contour lines are widely spaced.