Geophysical survey techniques are the most commonly used method of site identification when a suspected archaeological site lies beneath the land surface. They are non-intrusive and are subsequently invaluable when excavation is not possible.
Before conducting either a Geological Survey, or a Ground Survey, a grid needs to be created for the area under investigation. Control points are established which allow the site to be synchronized with the National Grid. First of all, a Baseline is established in the prevailing direction of the site. The ends of this are co-ordinated by Triangulation.
This fixes the baseline to the surround area, usually by using tape, which is then broken up into suitable units marked by stakes placed in the ground. Once this has been completed points perpendicular to the baseline are set up, again using tapes. In large areas, Pythagorean theory is implemented (A2 + B2 = C2) but when the area being surveyed is smaller, the Pythagorean triangle system is best.