Height References
The aim of this tutorial is to introduce Bench Marks as a height reference.
OS Bench Mark (OSBM).
OS
Bench Marks are established by the Ordnance Survey to provide height references.
They are usually carved into stonework or other stable material that is
unlikely to be disturbed.
The centre of the horizontal groove is the height reference.
Heights are given in metres above OS Datum on large scale OS plans and other references.
Temporary Bench Marks (TBM)
Temporary
Bench Marks are often established around the survey site. TBMs may be
surveyed in to the OS Datum by levelling between the site TBMs and an
OSBM.
A site datum may be established instead and all levels referred to a TBM that has been given an arbitrary value (usually 100.000 metres, or a value that ensures all heights will be positive).
TBMs require to be stable. The main site reference is often a steel pin set in a block of concrete but wooden pegs set in concrete with a nail head providing the reference level are often used.
It is good practice to establish a number of TBMs around the perimeter of a building site as a precaution against the only site height reference being disturbed or dug up part way through the contract.
