An Ordnance Survey Bench mark (BM) is a survey mark made by the Ordnance Survey to record height above the Ordnance Datum.

contour lines

A reduced level is the vertical distance between a survey point and the adopted level datum. There are two methods for calculating reduced levels, namely the "rise and fall" method and the "height of collimation" method. The latter reduces levels relative to the instrument height. As it has inferior built-in checks, it is unreliable.

A reduced level is the vertical distance between a survey point and the adopted level datum. There are two methods for calculating reduced levels, namely the "rise and fall" method and the "height of collimation" method. The latter reduces levels relative to the instrument height. As it has inferior built-in checks, it is unreliable.

a survey