Historical information

A prismatic compass is a common navigation and surveying instrument which was used extensively by the Forests Commission
The compass most commonly used to run a traverse line. The compass calculates bearings of lines with respect to magnetic needle. A magnetic deviation correction was needed to convert the reading to a true bearing
The compass could also determine angles between the traverse and waypoints (such as hilltop features).

Significance

Commonly used for navigation.
Largely replaced by modern GPS technology.

Physical description

Ex Army WW2 prismatic compass with leather case and strap.
Rugged design and made of metal the internal compass plate floats on oil to help it stabilise
The user lines up the direction of travel with the hair sightline in the fliptop cover and readings are taken through the viewfinder prism (hence the name).