100 link steel surveying chain, 1784.
© Science Museum / Science & Society Picture Library
This 100 foot chain was made by Jese Ramsden for use in the Primary Triangulation of Great Britain, the foundation for the first Ordnance Survey maps. It is shown with a plan showing the situation of the first measurement, a base line on Hounslow Heath measurement on Hounslow Heath (plate from 'Philosophical Transactions'). The first stage took place in the summer 1784, and was conducted by General William Roy (1726-1790) of the Royal Engineers.