Contour surveying is actually ridiculously easy, but because it involves a dumpy level, some record sheets, a pencil and (occasionally) a roll of string, most people get frightened about it and would rather pay out thousands of quid for somebody else to do the job for them. Here’s how to save yourself a lot of money.
First of all whack a peg into the ground where you want one end of your baseline to be. It’s best to choose the highest spot possible for this because, from this point onwards it’ll be downhill all the way. This peg will now become your TBM (otherwise known as the Temporary Bench Mark).
In order to find the exact height above sea level for the TBM you first need to follow the instructions set out in our article entitled ‘How to be an Archaeologist: Spot Levelling’. If you don’t know where that is then try looking under the ‘Previous Articles’ links in the right hand column.
Right, let’s assume you’ve established the height of your TBM. The next step is to get your baseline in place. This is accomplished by asking somebody to stand at the other end of the designated area with the measuring staff, using a compass to find the alignment, and then asking them to move about a bit until the staff appears in the dumpy lens.
It really is that easy.
Whack another peg in the ground where the staff is standing and connect it to the first peg with a long piece of string, as the following illustration demonstrates:
It’s important to ensure that the string is perfectly level otherwise any measurements taken from where the ground is uneven will vary from the TBM. (And if you didn’t understand that, then don’t panic. Just trust us for now…we know what we’re doing.)
Now, using your wooden stakes (or if you’re cheapskates like us, your tent pegs) mark out the divisions along the baseline at regular intervals. The size of these intervals will vary depending on the amount of land you’re surveying. For example, a site thirty meters in length might be suited to one peg every one meter, whereas a site three hundred meters in length would be better suited to one peg every ten metres (unless, of course, you want to spend the rest of your life stuck in the same field with only your dumpy for company).
For reasons of metaphor, we’re now going to call the baseline the ‘spine’ of the site. (This was actually a phrase coined by Chris Clayton. It’s a good ’un so we’re nicking it.) In order to establish the contours you’ll also need ‘ribs’ emanating from the ‘spine’ at right angles, determined by the position of the pegs/stakes that you’ve just smashed into the ground.
These ‘ribs’ should all be numbered for the record, using a combination of the number of pegs north or south of the TBM (if your baseline is running East/West rather than North/South then these will be east or west of the TBM). That’s probably got you confused again, so how about another visual aid?
Right…I reckon that should be about as clear as mud.
Now, from each peg along the baseline/spine, create a right angle with your dumpy by lining it up along the baseline/spine and then turning the level by 90 degrees, locate the measuring staff again in the same way that you did to create the baseline/spine in the first place, whack in another peg and join the two together with a piece of string. (It’s easy when you think about it…honest.)
Now, take a good look at what our mediaeval peasants are up to in the illustration. As you can see, one of them is observing the stadia pole (or measuring staff) through the dumpy, whilst the other is moving backwards and forwards up and down the slope. (Or at least he would be doing if the illustration was animated. Unfortunately it isn’t so you’ll just have to use your imagination.)
That’s because, having decided on the distance between the contours you want to survey (for example intervals of one meter) the staff-carrying peasant simply needs to locate the position along the ‘rib’ where that particular contour point occurs. This, of course, is the height of the dumpy plus or minus the additional measurement you’re looking for.
Alternatively you can just run a ruler/tape measure from the piece of string down to the ground, and when you hit the height of your contour (one meter using the current example) you can whack a peg in as a marker.
(If it’s still confusing try having a brew and then launching youself into another attempt to work out exactly what’s going on.)
Once the position of the contour is located, it’s a simple case of measuring the distance from the baseline/spine peg (in the illustration that’d be N1) to the point at where the measuring staff intersects the piece of string on the ‘rib’…or alternatively the marker peg you’ve already hammered into the ground.
This is then recorded on a sheet of paper and our mediaeval peasant moves backwards again until he reaches another meter in depth before taking the next measurement. (It doesn’t have to be a meter, of course. It all depends on what intervals you want your contours to be set at. If your earthwork, for example, is only ten centimetres in height/depth then one-meter contours would be a fat lot of use and the end result would be a blank piece of paper.)
When every single measurement has been obtained in this fashion, you then transfer the data to a sheet of graph paper. Well…actually that’s not strictly true. You cover a sheet of graph paper with a sheet of acetate and transfer the data onto that using the graph paper beneath as a guide.
(Don’t forget, of course, to take the measurements for the area west of your baseline/spine as well…or if your baseline/spine is running east/west to start with, make sure that you map out the contours north and south of it.) When you’ve done you should have an accurate contour survey.
Here’s one we prepared earlier, showing the agger and ditch at High Gate Lane in Stalmine, just to prove that it’s possible: