Retriangulation of Great Britain
The Retriangulation of Great Britain was a triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps of Great Britain. Data gathered from the retriangulation replaced data gathered during the Principal Triangulation of Great Britain, which had been performed between 1783 and 1851.
The Crow Knowl triangulation station at Crompton Moor in the South Pennines. It is one of the concrete pillars erected by the Ordnance Survey during the retriangulation of Great Britain. It was possible (in clear weather) to see at least two other trig points from any one trig point.
A Tavistock theodolite
Triangulation (surveying)
In surveying, triangulation is the process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline by using trigonometry, rather than measuring distances to the point directly as in trilateration. The point can then be fixed as the third point of a triangle with one known side and two known angles.
Triangulation of Kodiak Island in Alaska in 1929.
Liu Hui (c. 263), How to measure the height of a sea island. Illustration from an edition of 1726
