What is a Benchmark?
Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
Using the bench-marks.org.uk website, we can ascertain that levelling across the UK took place as follows:
- First primary levelling, England & Wales (1840-60). (184 lines)
- First primary levelling, Ireland (1839-43). (54 lines)
- First primary levelling, Scotland (1844-60). (108 lines)
- Second geodetic levelling, England & Wales (1912-21). (118 lines)
- Second geodetic levelling, England & Wales (1946-51). (14 lines)
- Second geodetic levelling, Scotland (1936-52). (81 lines)
- Third geodetic levelling, England & Wales (1950-68). (169 lines)
- Third geodetic levelling, Scotland (1956-68). (90 lines)
- Re-levelling of Greater London (1931-34). (21 lines)
- Cyclic relevelling, England (1956 onward). (0 lines)
- Cyclic relevelling, Scotland (1959 onward). (0 lines)
- Geodetic Levelling of Gibraltar (1961). (1 line)
A series of further levelling took place, using small local benchmarks. There is no documentation for these other than on older maps. Majority of benchmarks in my database are lower order. I only add a benchmark into this database once I have searched and either found, or not found it. Those benchmarks which are higher order are denoted with their line level (1GL for example).