Records from the second and third geodetic levelling of Great Britain by the Ordnance Survey. These books contain the data gathered in the field.
There are several series:
The hand-written A series contain descriptions and altitudes of bench marks levelled between 1912 and 1952. There is almost a complete set, although some were destroyed by enemy action in the Second World War.
The M series are the typed version of the A series with the addition of map references and details of the levelling back and forth from the bench marks. The series relates to levelling which took place between 1945 and 1954 and is complete.
Abstracts for the third geodetic levelling of 1951-1956.
Each volume relates to a polygon, the area of which is shown on the levelling route maps found in OS 57.