See Morris Hill

[agentmancuso]

I'd forgotten all about that mark Rob - from the picture, it appears not to have a datum line?

http://trigpointinguk-photos.s3.amazonaws.com/068/P68461.jpg

[Sagina nivalis]

Good point Bernie - no it apparently doesn't although this didn't strike me at the time. I'm amazed I even noticed the Mark!

[Sagina nivalis]

I'd forgotten all about that mark Rob - from the picture, it appears not to have a datum line?

Actually a Pivot Benchmark, it seems. There's a bit of a recess where a normal BM has a line. Neater one pictured here: http://www.bench-marks.org.uk/bm33472

[agentmancuso]

Makes sense. I've never seen one of those before, though I did look unsuccessfully for one down by Strathaven a while back.

[j.i.wilson]

I agree - this is a pivot bench mark - not easy to spot - well done!

The mark is used by putting a ball bearing in the pivot hollow at the head of the BM. I do know it is critical what size ball bearing is used - but I cannot remember what size is required!

They were used in the early days (50's, 60's mainly) on horizointal or near horizontal surfaces. They can often be found on the plinths of monuments.

Cheers for now

Ian Wilson

[Sagina nivalis]

I agree - this is a pivot bench mark - not easy to spot - well done!

Not me I'm afraid - analysis by Roger Templeman. Accuracy must be seriously compromised once the mark becomes worn.