In the past, a network of triangles covered the area to be surveyed. Points of reference with known positions were situated at the corner of each triangle, allowing the surveyor to take detailed measurements. In Great Britain, these fixed points are marked by the four-feet-high concrete pillars often seen on hilltops or in the corner of fields. A theodolite - which measures angles between points - was fitted on top of the pillar to determine their relative positions.
Having established control, the surveyor fits the details on the ground into the network provided by the control points. The calculation of surveys takes account of the different accuracy and methods of survey and control.