How to be an Archaeologist: Spot Levels and Benchmarks (Part Three)

Okay…hands up any of our readers who managed to solve the problem of how to take the ‘spot level’ reading at point ‘A’ from point ‘D’ as encountered at the end of the second part of this article? (Actually, hands up any of our readers who are still reading this? On second thoughts, don’t bother. Let’s push on.)

Right, here’s the solution.
But first the map.

Now then…we’ve already established over the last couple of postings the TMB for point ‘D’, which, incidentally, was 4.595 metres above sea level. Unfortunately, the terrain between point ‘D’ and point ‘A’ (the latter of which is the point that we’re trying to obtain a ‘spot level’ for) consists of a steep bank. The sort of bank that dumpy’s can’t see over. Putting the measuring staff on the peg at Point ‘A’ means that there’s no way the dumpy is high enough to read any measurements.
The solution? Put the dumpy over the peg at Point ‘A’ instead, and the measuring staff on the peg at point ‘D’.
Obvious really.
On the off chance that you didn’t quite follow that, here are a couple of diagrams that should help explain matters. Firstly, the wrong way to do it:

Secondly, the right way to do it:

With the dumpy and the measuring staff now in reverse positions, it’s a simple case of taking a reading from the staff and adding it to the height above sea level as recorded by the TBM (or, in this instance, point ‘D’.)
Let’s do that then. According to Ivan and Steve’s calculations, as we’ve already mentioned, point ‘D’ was exactly 4.595 metres above sea level.
The measurement on the staff as viewed through the dumpy was 3.31 metres.
The total height above sea level was, therefore, 7.905 metres.
So the height of point ‘A’ is 7.905 metres above sea level.
Or is it? Anybody spot what we’ve missed? That’s right…we haven’t taken the height of the dumpy itself into account. So, one last measurement of the dumpy then, which on this occasion came to 58 centimetres. Take this away from the total height and you have…7.325 metres…or to put it another way, 7 metres 32 and a half centimetres above sea level.
Okay…did all of that make sense?
I hope it did, because that’s the simplest use of the dumpy that there is.
In future articles (although I can’t promise you when yet) we’ll be covering ‘contour surveys’ and ‘gridding out’, both of which require the surveyor’s level and both of which are absolutely essential when it comes to correct procedures for conducting archaeological surveys. In the meantime, one last photograph of Steve and Ivan on the riverbank at Cockle Hall, with Steve proudly showing off his extended staff…