Editing Retroreflector (section)

===Surveying===
[[File:Prism Target used with survey equipment (cropped).jpg|thumb|upright=0.7|A typical surveying prism with back target]]

In [[surveying]], a retroreflector—usually referred to as a '''''prism'''''—is normally attached on a [[Pole (surveying)|surveying pole]] and is used as a target for [[distance measurement]], for example, a [[total station]]. The instrument operator or robot aims a [[laser]] beam at the retroreflector. The instrument measures the propagation time of the light and converts it to a distance. Prisms are used with survey and 3D point monitoring systems to measure changes in horizontal and vertical position of a point.
Two prisms may also serve as targets for [[angle measurement]]s, using total stations or simpler [[theodolite]]s; this usage, reminiscent of the [[heliotrope (instrument)|heliotrope]], does not involve retroreflection per se, it only requires visibility by means of any source of illumination (such as the sun) for direct sighting to the center of the target prism as seen from the optical instrument.