The Ordnance Survey have been mapping the UK for over 200 years. Until recently they used simple mathematics, theodolites and a national network of benchmarks and trig points to achieve one metre accuracy, a triumph of engineering in its day. That technology has now been replaced by more accurate satellite navigation and the physical position markers are redundant.
The best known of these markers are the trig pillars, a common sight when walking in the country. These were built to last but they are decaying slowly. Other markers such as bolts and surface blocks are more easily damaged. The village of Bookham was served by three surface blocks, all now vanished, possibly just buried a few centimetres below the ground, or maybe dug up and destroyed.
The background notes explain how to find the trig points and benchmarks close to you.
This talk was originally given on behalf of the Surrey History Meetup