On a clear, crisp spring morning in 1936, a group of men gathered around a strange, pale obelisk in the middle of an unremarkable field in Cold Ashby, Northamptonshire. Those men were there to begin the greatest undertaking Ordnance Survey had attempted since the early 19th century.
That shining white monolith would now be instantly recognised by any walker, hiker or geography pupil. It was of course a Trig Pillar, and today, 18 April, marks 75 years since the day when they were first used in anger at the beginning of the Retriangulation of Great Britain.
Trig Pillars now evoke the kind of sentimentalism of something quintessentially British (although there are equivalents in other countries), but at the time they were part of a state-of-the art network built to literally re-write the map of Britain. The Retriangulation was an enormous task and lasted up until 1962 (with a break for World War II), the impact of which we still live with today.
Triangulation is basically a mathematical process that makes accurate map making possible. It works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline and in this case, those known points were the 6,500 Trig Pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the Retriangulation, many rounds of angles would have been measured with the observations taking several hours.
But why was the Retriangulation needed?