Transferring Datums

Temporary Bench Mark (TBM) (TBM) from which levels are taken A Temporary Bench Mark (TBM) is a known height point from which all levels are taken. The TBM can be related to an Ordnance Survey Bench Mark. The TBM can be surrounded by concrete and a timber enclosure for protection.

Ordnance Survey Bench Mark Level taken from here Ordnance Survey Bench Mark – datum which marks the height above mean sea level at Newlyn in Cornwall. These datum's are established by the Government Survey Office and are usually permanently marked on public building, bridges etc.

Example of Ordnance Survey Bench Mark

A Datum peg is a level point that is established in relation to a Temporary Bench Mark. The datum is a level reference point for the building activities, a few are shown below. Datum peg at DPC level DPC Oversite concrete level Foundation concrete level Invert level

Using Straight Edge and Level Transferring Datums Using Straight Edge and Level

A B X Drive a peg (X) in the lengths of the straight Mark the position of the level on the straight edge to ensure it remains in the same position. B Drive a peg (X) in the lengths of the straight away from the datum and level. X

B X A Y To maintain accuracy the spirit level and straight edge should be reversed at each intermediate leveling point as seen point B on the straight edge remains on peg X and A swings round to peg Y.

A B A

Any inaccuracies in the level and straight edge are corrected by reversal of the level and straight edge as illustrated below. TBM Peg 1 Peg 2 Peg 3 Peg 4 Datum Level