“What datum? GPS or Ordnance Survey?”

Have you ever been involved in a project and found your datum doesn’t agree with the adjacent development and yet both claim to be based on OS Datum? You may not know that the last time the Ordnance Survey checked a benchmark was over 30 years ago and the heights should not be relied upon to accurately define OS Datum.

All benchmarks are subject to natural and accidental disturbance through reconstruction and subsidence; consequently values become inaccurate and require revision. This is normally manageable provided the benchmark infrastructure is maintained. As this situation no longer exists the whole OS benchmark network is now in decline, this has led to other agencies establishing their own networks determined by high precision GPS techniques. The OS has also remodelled its level network using GPS.

For the unwary this can lead to confusion when selecting the datum for a new project when the choice is between a GPS network, OS benchmarks or a local definition. This does not mean that one or other is less accurate or in error. What is important is that all the levels on a particular site are consistent with the datum used for that site however it was defined.

Please call us if you require further advice on which datum is appropriate for your site or development.