Detail from OSD 2, Mounts Bay, Cornwall. Showing gun emplacements on St. Michael's Mount.

This detail is from the coastal survey showing the defence works of Mount's Bay, from Mousehole to Marazion. The area's status as a key defence point is shown by the large scale of the map: 6' to the mile. Positions and weights of cannons around the bay are noted, and the artillery on St Michael's Mount is described by a key. Small crosses mark the positions of rocks in the bay. The irregularly protruding surfaces of Augusta Rock and Long Rock are indicated by intersecting lines of varying lengths, differentiating these from the sand, which is symbolised by dots. The condition of the drawing is excellent. It is coloured with watercolour washes.

Find out more about the history of the Ordnance Surveyors Drawings in our On the Trig Themed Tour.