Hilary wrote:

Something the meaning of which eludes both me and a friend is an odd
sign which we have spotted on things as diverse as old buildings -
colleges, churches, and mileposts. It looks not unlike an inverted
War Department sign stamped on any piece of WD cutlery - that's going
back some, and the cutlery was purloined by my father when he was in
the RAF, so don't blame me!

It has a straight horizontal line, central to that is a line dropping
downwards at 90 degs, and either side of this central line are lines
at 45 degs. If you take the points of the compass, the main line
would read W to E, the central line (which meets but does not cross
it N) to the S, and the lines either side ESE and WSW (I think!).

They usually seem to be carved in stone, and we thought at first they
might be the mason's mark, but there is a diversity of age and
situation - some in Oxford, some in Huddersfield and Manchester. Any
light to shed on the subject?

Hi Hilary,

They are Ordnance Survey "bench marks" which are precisely surveyed for their height above the zero level for the contours seen on ordnance survey maps. Their height is measured using precise methods and is accurate to about a cetimetre.

Like the "Trigonometrical Points" that you see on the tops of many hills, which are accurately surveyed in plan position (latitude and longitude, and/or grid reference), they are largely being supplanted by Global Positioning Systems (GPS) that use the same orbiting satellites as sat-nav systems in some expensive cars - and motorhomes!

Hope this helps!

Tony

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