Dr. Gaskell calls what he does “stereophotoclinometry.” Ideally he needs at least three images of the target landscape, usually taken by an orbiting spacecraft or a probe on a flyby to another destination. Only in rare cases can telescope images provide enough detail.

The sun angle must be different for each exposure so each image shows different shadows. By comparing the shadows, the software calculates slopes, which yield the altitudes of target features. The computer solves the equation in three dimensions, producing a patchlike topographical maplet.