Ordnance Survey benchmarks are survey marks made by Ordnance Survey to record height above Ordnance Datum. They can be found on walls and buildings across Britain and were a way of recording height at a given point. If the exact height of one benchmark is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. The term benchmark originates from the chiselled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a levelling rod, thus ensuring that a levelling rod could be accurately repositioned in the same place in future.
**500,000 benchmarks** created, but this number is reducing as roads change and buildings are demolished. Greater London has almost **18,000 benchmarks** as well as large clusters in Leeds, Bradford and Birmingham².