B.M. (Ordnance Survey Bench Marks)

I am slowly adding the Ordnance Survey Bench Marks or OSBMs. They are indicated as B.M. with a height on the old early 1900s Dartmoor Maps. It is surprising how many of these can still be found. However, as with all Dartefacts, an item shown in these maps may not exist! If I have visited a possible location I will still show a photo, but will add (NOT LOCATED) instead of a height in metres. Bench Marks were used to record height above 'Ordnance Datum'. If the exact height of one B.M. is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. OSBMs are often found on buildings or other semi-permanent features but on Dartmoor boulders, standing stones (!), field walls, gateposts etc. are often utilised. Although the main network is no longer updated the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Some Bench Marks also have a 'bolt' inserted above them. I am starting to indicate which direct the B.M. 'faces' as this can aid discovery. In order to keep the B.M.s in an ascending height order I will have to add a '0' to the beginning of the height numbers i.e. 'B.M. 0648.5', although on the old maps it is written 'B.M. 648.5'. Thanks for that WordPress. Thanks.

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