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surely every bench mark will be > correct Above Ordnance Datum regardless of which datum, Liverpool or > Newlyn, was originally used? Yes the information held by the Ordnance Survey should be correct but that doesn't mean the number written on the 1921 map you have on your desk will be what it is now. It should be the level at the time of the publication but will often say something in the notes like "resurveyed 1918 - Relabelled 1912" Also worth noting not all of a plan was necessarily resurveyed at the same time. There are plenty of examples of features on one part of the map not continuing onto the rest since they date from after the survey of that section. An example I recall is buildings that were demolished prior to the construction of a road which appear on a map with the road on another section, implying the road was well under construction (even part in use) before the houses were taken down. If you are looking at maps for evidence of shafts in operation at the same time, or cottages which might be associated with a mine, be careful. I believe OS use satellites etc now and most benchmarks are redundant (so are triangulation pillars) Regards David P.S. (I'm a chartered building surveyor)