How to Set a Datum Point on Site With a Laser Level

Every accurate build starts from a datum — one fixed, known height that every other level on the job is measured against. Get the datum right and everything downstream (foundations, floor levels, drainage falls) lines up. Get it wrong and the error carries through the whole site. Here's how to set one properly.

What is a datum point?

A datum is a permanent or semi-permanent reference point of a known height. On large jobs it's tied to an Ordnance Survey benchmark or a Temporary Benchmark (TBM); on a domestic job it might simply be a solid peg, a manhole cover, or the damp-proof course of an existing building. The important thing is that it won't move and everyone works from the same one.

Step 1: Choose a stable reference

Pick something that can't be knocked or driven over — a concrete-set peg, an existing structure, or a securely bedded stake. Mark it clearly and record its level. This is your 0.00 reference.

Step 2: Set up your rotary laser

Position a rotary laser level on its tripod where the beam can reach the whole working area, and let it self-level. A kit like the Grettz G60 gives you a full 360° reference plane to work from.

Step 3: Read the datum height

Stand your levelling staff on the datum and slide the receiver until it reads on grade. Note the figure — say 1.250 m. That reading now represents your datum level. Everywhere else on site, the same staff reading means the same height.

Step 4: Transfer levels around the site

To set a new peg to the same level as the datum, move the staff to the new position and drive the peg until the receiver reads 1.250 m again. To set a point higher or lower by a known amount, simply adjust the target reading. For example, a point 150 mm below datum would read 1.400 m on the staff.

Step 5: Protect and re-check your datum

Re-check against the datum at the start of each day and after any knock to the tripod. If you need falls or slopes rather than dead level — for drainage or a patio — you'll want a dual-grade laser such as the G70 or G80, which can project a set gradient rather than a flat plane.

Once your datum is set, setting out the rest of the job is straightforward — see our guide on setting out a foundation with a rotary laser.