In a widely used analog procedure the two photos are placed in the projectors of a stereoplotting instrument. With the aid of visible ground-control points the photos are oriented to the relative positions they had at the instants of exposure. In a typical automated stereoplotting system  scanning devices substitute for human eyes to sense model-surface slope and thus to control servomechanisms that raise and lower the plotting table and translate it along a succession of closely spaced parallel horizontal dimensions, or ground profiles.