Sunday, March 16, 2008

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

Last week we established (more or less) how to work out the height of the crosshairs on your dumpy (or surveyor’s level) above the low water mark by using a permanent benchmark. Once we have this information we can now transfer it to what’s known as a ‘spot level’ or ‘temporary benchmark’ some distance away.
Let’s have our map up again.

Now then, point ‘A’ on the map above is the location at which we want to determine the height above sea level. Point ‘B’ is the current location of our dumpy, next to the benchmark on the signpost. (All right…any of our readers who tuned in a few days ago will know that there isn’t actually a benchmark on the signpost, because the real benchmark is missing. But for the sake of this exercise we’re assuming there was.)
Unfortunately the terrain between point ‘A’ and point ‘B’ looks like this:

To put it another way, trees, brambles and a steep bank obscure the line of sight between our dumpy and the point we’re trying to survey, so we’re going to have to take this in stages.
Our first objective then is determine the height above sea level at point ‘C’ on the map, which isn’t obscured by any obstacles (other than a few dog walkers).
So, a peg is hammered into the ground at point ‘C’, the Eastings and Northings are taken on the ‘Sat Nav’ (just for the record) and the measuring staff is balanced on the top of the peg.
Now, assuming that we haven’t moved the location of the dumpy
since last week, we simply swivel the top of the dumpy around and focus in on the staff. Where the crosshairs meet, we take a measurement. In this particular instance it was 1.465 metres.
When subtracted from the height of the dumpy above sea level (which we worked out, but didn’t include the figures for, last week) that gives us a total height of 4.595 metres.
Now you have a ‘spot level’ or ‘temporary benchmark’ measured
from the peg at point ‘C’. Write this down in your notebook and stop for a cigarette/butty/hot brew.
Before we move on, however, you might also want to record the distance between points ‘B’ and ‘C’. In this particular instance, that would be a simple case of stretching a tape measure between them. Where the terrain is hillier, or the distances are too great for the average tape measure, however, the mathematical laws of perspective come into play. Here’s what you do.
If you look through the viewfinder on your dumpy, you’ll probably see something remarkably similar to the what’s being shown in the photograph below.

You might have noticed the horizontal lines situated above and below the crosshairs. They’re called stadia lines. (Again, don’t ask us why because we honestly don’t know. They just are.) Keeping both the dumpy and the measuring staff in exactly the same positions they were in when you were taking the ‘spot level’ for point ‘C’, measure the distance on the staff between the upper stadia line and the lower stadia line.
In the example shown in the photograph that’s roughly 12 centimetres.
Now multiply that measurement by 100. Again, in the example, that would be approximately 12 metres.
That, quite simply, is the distance that the dumpy is situated from the staff, and therefore the distance from the Temporary Benchmark (or TBM).
The formula is simple, multiply the distance between the stadia lines as measured on the staff by 100, and you get the distance between the staff and the dumpy. It really doesn’t get much easier than that.
Right, time for our next ‘spot level reading’. At this point we have to move the dumpy and reposition it carefully over the peg at point ‘C’. In order to be absolutely accurate, a plum line is hung from beneath
the dumpy to the top of the peg. It’s also important to remember that the dumpy needs to levelled again using the spirit levels, which, by way of consequence means that the legs on the dumpy are extended and/or contracted, according to the make-up of the ground. This, understandably, alters the dumpy’s height, so this needs to be measured using your tape measure as it has to be taken into consideration.
The dumpy’s height is added to the height above sea level as recorded at point ‘C’, giving you the height of the dumpy at point ‘C’ above sea level. The measurement is then read from the staff at point ‘D’, which in turn is subtracted from the overall height that you’ve just established. This should give you the height of the ground above sea level at point ‘D’.
Okay…so far so good. We hope everybody’s following this because, if you’re not, there’s not a lot we can do about it. The photograph below shows Ivan and Steve struggling to hammer a peg into the ground at point ‘D’.

Unfortunately, the going from here’s a bit tougher. The final leg of our journey is uphill and point ‘A’, somewhat annoyingly, is considerably higher than the viewfinder on the dumpy. This means that when the staff is placed on the peg at point ‘A’, the dumpy will be too low down to be able to read it. We've also run out of space, so the solution to our problem will have to wait a few more days.


Feral Beast said...

Thanks for telling me that Brian.

Brian Hughes said...

No problem Mr. Beast...I'm just amazed anybody actually bothered to read it.