control point

Permanently marked locations that are used as reference points for survey or mapping work. The coordinates of a control point are determined with a high degree of accuracy using terrestrial and/or satellite surveying techniques. The coordinates may be expressed in terms of a satellite reference coordinate system (such as with respect to ITRF), or a local geodetic datum.

Control points are established, marked, surveyed, and documented by a statutory authority for a city, state or country, or perhaps by a construction company within an engineering site. The known location of a control point can be used to set up a total station, for example, to locate points of detail by radiations. Another control point might be used to determine the orientation of the coordinate system by measuring some angles.

Control points are permanently marked using a metal bolt, plaque, or other type of permanent monument. Permanence is required so that the point is easily located using the provided documentation, and so that the point is stable and therefore the coordinates are reliable.

Control points are also called control stations.