Thursday, June 05, 2008

How to be an Archaeologist: Contour Surveying

Before you can start any official archaeological excavation you first need to contour survey the area designated for the dig. There are several reasons for this. One: To get the lie of the land as a guide to the best placement for the trenches. Two: So that the trenches can be accurately placed on Ordnance Survey maps, making it easier for future archaeologists reopening the site to locate them. Three: Because it looks good in the excavation report.
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 choos
e 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 l
evel 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 w
est 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 dow
n 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:

19 comments:

chris2553 said...

Ah, so that's what we were doing.

Brian Hughes said...

Give or take the mediaeval garb...

Jayne said...

Got the dumpy level - check.
Got the string - check.
Don't got the medieval skirts and mob cap... so I'll be sending FB over to you, Brian, for the authentic learnin' :P

Brian Hughes said...

Jayne,

The mediaeval clothes are optional...although it can get a bit chilly surveying in the nuddy in the middle of the winter.

Feral Beast said...

Thanks for letting us know about that, I really enjoy learning.

Brian Hughes said...

Mr. Beast,

This article is a lot easier to follow, of course, if you actually own your own dumpy level, have a convenient field to use it in, and get in plenty of practise. That's always the best way to learn these things. Ask your mum to buy you one and then you can spend hours and hours showing her and your dad how to use it. (She's going to thank me for this...I can tell.)

Jayne said...

Ummm Brian? We already have a dumpy level, seriously...we've just misplaced the handy paddock in which to use the damn thing :P

Brian Hughes said...

Jayne,

Well there you go! Get the feral one to survey his excavation near your 'chook' house. You'll have to give him a hand, of course, because it takes at least two people to conduct a contour survey. Every site tends to be different, so it's a good exercise in the old grey matter. By the time you've finished, though, your back garden'll be one of the most professionally recorded sites in Australia. (And, it'd make for an extremely useful addition to the feral beast's C.V. when he comes to apply for that all important university placement.)

Jayne said...

LMAO
A great whopping hole in the ground is his dig at the moment, although it is giving up quite a lot of finds.
Will be interesting to see how we manage to plot around the walled vegie garden boxes and the Hills Hoist LOL.

Brian Hughes said...

Jayne,

All archaeology is whopping great holes in the ground...unless it's underwater archaeology, of course, in which case it's a whopping great hole underwater. The difference between just a hole in the ground and professional archaeology is the excavation report. Well, that and the hat with a feather in it...

Jayne said...

Arrrg, Oi see!

Just checked the Time Team website, holy sheep dung we've got a stack of stuff to catch up on here in Oz !!!

Brian Hughes said...

Jayne,

Most of it is Phil Harding in his ghastly shorts and the recession of Tony Robinson's hairline diametrically opposing the increase of Mick Aston's girth.

Jayne said...

I couldn't help but notice the growing picture of Mick Aston in the March/April issue of British Archaeology, which is pretty harmless along with Tony Robinson's disappearing hair.
It's those shrinking shorts on Phil that are the real worry for viewers and the censor at the BBC!

Jayne said...

Just checked the discounted price tag on the laser level and I told a lie.
We didn't pay $5, we paid $6.
And we checked it again today, works like a dream :)

Brian Hughes said...

Six Australian dollars? That's (calculates quickly on fingers, with tongue sticking out of side of mouth in concentration) about 24 pence sterling. That dumpy of ours cost over a hundred quid (or to put it another way, four billion Australian dollars)! Well, when I say 'ours', it was Steve who actually bought it. Our other dumpy's on loan from the museum. No, it is, seriously. I'm not joking. Hence the meadiaval peasants. You're going to have to send me the catalogue you're using.

Jayne said...

It was one of those lucky finds in the returned products/discounted products bin in our local Reject Shop
http://www.rejectshop.com.au/index.php
The outside packaging was missing and it's missing one of the plastic "window" panels from the box.
Haven't seen them stock anything like it since - not that we'd seen them stock the laser level in the first place at all!
If we trip over anything like it again we'll grab it and post it to you ;)

Brian Hughes said...

That's what I call a lucky find, Jayne. We had to order ours from York on the other side of the country.

Ozfemme said...

Just a quick question - what colour should the string be?

Brian Hughes said...

Bella,

The same colour as my eyes. White with red streaks.