Victorian illustration to download showing two soldiers of the Royal Engineers in an observatory high above the ground making observations for mapping purposes using a theodolite. One soldier operates the theodolite while the other notes the results in a book. They wear uniform – short jackets with epaulettes and trousers with a stripe down the sides. The small hexagonal observatory is made of wood with a canvas roof. This observatory was built on the top of St Paul’s Cathedral in 1848 as part of the Ordnance Survey’s project to map Great Britain at a scale of 6 inches to a mile.