The Ordnance Survey have been mapping the UK for over 200 years. For most of that time they used simple mathematics, theodolites and a national network of markers - benchmarks and trig points - to achieve accuracy that was a triumph of engineering in its day. That technology has now been replaced by yet more accurate devices using satellite navigation technology such as the Global Positioning System (GPS).
The physical markers are still there, but they are now industrial archaeology. The best known are the trig pillars, a common sight when walking in the countryside. 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 in Surrey 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 talk covers the history of the Ordnance Survey from the first complete survey of the UK in the eighteenth century through to the last one in the twentieth century. It describes how theodolite-based surveying techniques and equipment improved through that time until they were made redundant in the 1990's by satellite-based systems.
The background notes explain how the trig points and benchmarks were used and show how to find the ones near you.
The speaker Simon Ritchie works in IT and has been experimenting with accurate satellite surveying systems for several years. He also runs the Surrey History Meetup