How To Establish A Datum Level

It is important that all levels in a building are taken from a fixed point called a datum. This point should now be established; wherever possible it should be related to an ordnance benchmark. This is an arrow with a horizontal mark above the arrow as shown in below picture. The center line of the horizontal is the actual level indicated on an Ordnance Survey map. Benchmarks are found cut or let into the sides of walls and buildings. Where there are no benchmarks on or near the site, a suitable datum must be established. A site datum or temporary benchmark could be a post set in concrete or a concrete plinth set up on site.

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