Census Of Rookeries Volunteer User Survey
In 1927, the grid co-ordinate system used by the British for their military maps was superceded by what has become known, among other things, as the Cassini Grid (after the method of projecting the grid onto the curvature of the earth).
In turn, it was intended to replace the Cassini Grid with the National Grid; but this was delayed by the war and did not occur until 1947.
Although both systems utilise a 1 km grid, there is no simple means of converting a Cassini Grid Reference to a National Grid Reference. John Penny published a conversion formula which has been used here.
The 1944-46 National Rookery Survey was mapped using the Cassini Grid.
Penny, J. 2000. Straight to the point right on time. Regional Historian 5
Cassini Grid Ref
National Grid Ref
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