An even-better-than-usual essay by the invaluable J. B. Crawford:
[Brigadier Martin Hotine’s] greatest memorial is an artifact of his work: squat concrete pillars surmounted by a triangular brass plate. Found atop mountains and hills throughout the United Kingdom, these "trig points," designed by Hotine himself, are the physical references of the Retriangulation of Great Britain. This effort, spanning from 1935 to 1962 with the interruption of WWII, revised an original triangulation (initiated in the 18th century) as the basis for British surveying. Through the course of this effort, Hotine developed methods that would revolutionize the field of geodesy. His collaboration with mapmakers from the United States, a continuation of his wartime surveying for the Allied Forces, set the stage for one of geodesy's most ambitious projects: a measurement taken across the Atlantic Ocean.