Datum Level
/DAY-tum LEV-el/
Also known as: datum, site datum, temporary benchmark, TBM, setting-out datum, ordnance datum
Definition
A fixed reference height used on a construction site from which all other levels are measured. The site datum is typically marked on a physical peg or fixed point known as a Temporary Benchmark or TBM, and relates to Ordnance Datum Newlyn (mean sea level) or a convenient local reference height.
In Practice
Before groundworks begin, a datum level is established and pegged on site, usually by a surveyor. All excavation depths, foundation levels, finished floor levels, and drainage falls are set out relative to this point. On larger sites, multiple TBMs are established and cross-checked. The datum must not be disturbed during works - it is often set on an adjacent wall or boundary structure rather than a stake in the ground.