Sunday, March 16, 2008

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

Lately we’ve been trying (although not always with a great deal of success) to hammer home the message that it’s vitally important for all archaeologists (both amateur and professional alike) to conduct their excavations with an acceptable degree of consideration for what they’re doing. This includes proper and accurate recording techniques, stratification recognition, employment of context sheets and other invaluable tools of the archaeologing trade and not…I’ll just repeat that in bigger letters, ‘NOT’…just tearing into sites with no regard for what’s being destroyed.
Bearing all that in mind meet Ivan and Steve, possibly the most important members of the Wyre Archaeology Excavation team. It’s their unenviable job to contour survey, plot, square up, grid reference and spot level all of our excavation sites.

Without Steve and Ivan the rest of us couldn’t open up any trenches. Well…actually we could…but we wouldn’t be able to place them accurately on Ordinance Survey maps (or even our own excavation plans because, to put it simply, we wouldn’t have any) and future archaeologists returning to the scene of the crime wouldn’t be able to find them again.
In other words, Steve and Ivan (along with their dumpy level…don’t ask me why it’s called that…it just is) prepare the groundwork for all us diggers by creating the framework from which our excavation reports hang.
It’s an important job, and somebody has to do it.
So, in this article we’re going to show you how to take a simple ‘spot level’ reading, otherwise known as creating a temporary benchmark (or TBM). At some point in the future we’ll no doubt be covering the survey team’s other jobs of ‘gridding out’ and ‘contour surveying’, but it’s always best to start with the basics.
Every site plan begins with a particular point on the map, for which we’ll need to know the Eastings and Northings (otherwise referred to as the grid references) and the height above sea level. For the purposes of this illustration, we’ve decided to choose point ‘A’ on the map below, which is on the top of a steep bank overlooking the remains of Cockle Hall at Stanah.

Eastings and Northings are simply the grid co-ordinates on Ordnance Survey maps that help us determine the exact location we’re attempting to survey. A satellite navigation system should allow you to pin point the spot accurately enough on the ground. In this instance let’s take it as read that the co-ordinates for point ‘A’ are 336081/442085 and that our ‘location peg’ has already been firmly hammered into the top of the bank.
Now we need to work out the height of point ‘A’ above sea level, and to achieve this we first need to find a permanent benchmark.
By ‘benchmark’ we’re not referring to those misspelled phrases chiselled by teenagers into park benches. No…a benchmark is actually a mark that resembles a cubist’s attempt to draw a picnic table, left by the Ordnance Survey teams, generally on a gatepost or some other semi-permanent structure. The photograph below shows a typical benchmark minding its own business.

Benchmarks are also known as crow’s feet (possibly because they resemble the stretch marks around Helen Mirren’s eyes.) Their main purpose in life is to indicate the height above sea level at that given point.
They’re also included on Ordnance Survey maps. If you take another look at our map somewhere above, you’ll notice that in the lower right hand corner are the letters B.M. and the figures 5.49m.
This means that there should be a benchmark similar to the one in the photograph, next to the picnic site at Cockle Hall, and that the horizontal line on the top of the benchmark is exactly 5.49 metres above the low mater mark.
We say ‘should be’ because, in reality, there isn’t.
Sometimes benchmarks are recycled by farmers, fed up with draug
hts blowing in through their kitchen walls. Sometimes, such as the ones carved into houses, they’re plastered over. Sometimes the Wyre Rangers pull them down not realising what they are and cart them off to Fleetwood Tip, which is what appears to have happened at Stanah.
When this occurs, it’s usually not worth trudging several miles to find the next benchmark along, because, invariably, that one’ll have vanished as well.
Contour lines make a reasonable alternative, just so long as you know the exact location of the contour line on the map and can pinpoint it again on the actual ground. Unfortunately contour lines are seldom painted onto real hillsides. Making a note of the grid references on the map and then using a satellite navigation system once again to find the same spot in reality generally does the trick.
Another alternative for missing benchmarks are ‘spot levels’ that appear on Ordnance Survey maps. Our map somewhere above, for instance, has a spot level of 5.2 meters located close to the top of it.
Anyhow, for the purposes of this article, we ended up improvisin
g and drew our own highly inaccurate benchmark with a Biro on side of the guidepost at Cockle Hall picnic site. (We don’t recommend this for genuine archaeological surveying, of course, but this is for demonstration purposes only. We would have used the other ‘spot level’ included on the map, which would have been better, but I stupidly forgot to pack the map before heading off for Cockle Hall, so we couldn’t.)
Okay…once you’ve located your benchmark (or equivalent thereof) the next step is work out how high above sea level the crosshairs on your dumpy are.
This is easily achieved by using the metric staffs that come with the dumpy, and a few simple mathematical calculations.
The measuring staff is placed against the benchmark, the dumpy is levelled on the ground (using the built in spirit levels) and a height reading is taken by looking through the lens and making a note of where the crosshairs are aligned on the measuring staff.
The distance between the horizontal bar of the benchmark and the crosshairs on the dumpy level, when added to the height of the benchmark above sea level itself, give us the height of the dumpy above sea level.
Does that make sense?
Hold on, let’s include a diagram and see if that works better.

In effect, what you’re actually doing is measuring how high the crosshairs on your dumpy are above the benchmark. Because the benchmark is already set at a specific height above sea level, this measurement, when added to the established height of the benchmark, gives you the height of your dumpy above sea level. It’s logical when you think about.
Armed with this information (which is known, for reasons best left to surveyors, as the ‘back-sight’) you can now proceed to create the first of several Temporary Bench Marks, or ‘spot levels’, leading from the permanent benchmark to the actual location you want to record. But that’s probably enough information for the average reader to absorb in one week. We’ll continue this article, therefore, in a few days time.

14 comments:

John said...

Nice post! You kid around a lot, so its good to remind us just how serious, and professional, you can be.

In America, here, I was visiting the site of Fort Nonsense (yes, that's its name) and found a brass circle embedded in th grass that had a date of a survey done, and all the proper information recorded on it. If I can find the photo, I'll share it with you. Thanks for explaining its purpose to me.

One question: You didn't describe what a location peg is, did you?

Also, why does it always, eventually, come down to Helen Mirren?

Cheers, JOHN :0)

Brian Hughes said...

John,

A location peg is, basically, the same as a tent peg...in our case, a bright orange affair with a hole towards the flat end so that a string can be threaded through it for when the site is being 'gridded out'. Some archaeological surveyors prefer to use wooden pegs with a screw in the end that can be adjusted to get the levels right on a sloping baseline...if that makes any sense? If you watch the opening credits of Time Team, that stake they're driving into the ground with a string running from it is a location/spot level peg. In fact, in the next shot, there's somebody using a dumpy alongside it.

As for Helen Mirren...everything always comes down to Helen Mirren. It's her own fault for pretending to be the Queen.

Feral Beast said...

Mum has been teaching me about eastings and northings. Lets just hope that its not a vampire instead of the ground that they're driving a stake into.

Brian Hughes said...

Mr. Beast,

That'd certainly liven the survey up a bit. I've got my Van Helsing hat on standby just in case.

JahTeh said...

Off topic, Fleetwood but have you heard anything more about the discovery of a supposed Druid's grave in Stanway? It's a wooden burial chamber dating to 40-60 AD with cremated remains.

JahTeh said...

And I understood everything in this post. I use the same methods digging down through the detritus to the carpet when I springclean.

Brian Hughes said...

Witchy,

Short answer: 'No'.

Longer answer: 'Don't even known where Stanway is.'

Even longer answer: 'We also tend to sweep the winter's refuse under the living room carpet creating undulations and interesting features for which a five metre high extendable surveyor's pole just isn't long enough to measure.'

Brian Hughes said...

Slightly more practical answer: The following Internet address will probably be of interest to you. Just don't pay too much attention to the rubbish being spoken by the know-it-all archaeologist...

http://dsc.discovery.com/news
/2008/02/11/
druid-grave.html

Fiona said...

Brian, another description about dumpy's and setting up.
The below is taken from:
Bettess, F. 1998. Surveying for Archaeologists. Durham: Penshaw press, 39-42

A dumpy is also known as a 'level'.
Once out of the box, attach to tripod. Tripod legs are normally telescopic and should be extended fully when leveling, increasing its stability. (This can be ignored if it need to be kept low).

Now, set the instrument at the spot wehre you want to work, but do not set them firm in the ground. Look at the tripod and judge where it is horizontal, if not adjust the legs until satisfied. Now firmly press each leg into the ground (Betress 1998: 40).

The tripod should be roughly level, but if not use the footscrews (x3) so that the bubbble comes into the centre of the tube and is parallel with two footscrews. Now, turn the instrument 90degrees and using the thrid footscrew bring the bubble into the centre of the tube.

To check it is level, turn the instrument back to its original position and alter as needed.


Once the instrument is set up and is levelled with a benchmark (done by holding them bottom of the staff to the level of the benchmark. A reading is then taken through the telescope and the reading from the horizontal cross-wire is noted and added to the benchmarch level and the answer should be the reading of the cross-wire.

(If using an ordinance survey benchmark like the picture in Brian's article, the level is the horizontal line).

Now the 'dumpy' can be used for levelling.
The staff is held on the level point you wish to find. Take a reading from the staff. Subtract this from the level of the cross-wire you got before, and this will be the level of the point.

The staff should be held vertically - use the bubble as a guide - and should be stationary. Stand sideways to the staff (i.e. shoulder to dumpy).

DONE!!!!!

This probably won't help much but I thought I'd give ti ago. If you get practise at it, it becomes easy!! I know, I have the T-shirt and cap from a very wet day at Badbury Rings!!

The book also contains detailed info about how to record the values etc. It is a very good publication, and I shall bring it home with me other easter, for anyone who may wish to borrow it.

I am hopping to attend the April meeting. Is the date known??

Sorry for the essay - didn't know where else to put the information.

Fiona

Brian Hughes said...

Fiona,

That's more or less what I've written...only with less facitious remarks, of course, and a lot more focus on the spirit level bubbles.

Actually, there's another two of these articles about 'spot levels' to go yet...the only other useful hint amongst them being the use of the stadia lines. The rest of it is just so much waffle, but I have to fill this board up with something every week. (And I haven't mentioned the fact that Steve's Sat Nav takes spot level readings automatically...thought the traditional approach sounded more impressive.)

Not sure what date the April meeting's on...the third Wednesday whenever that is. (Haven't got my calander upstairs with me and I'm too lazy to go down and check it.)

Whenever it is, hopefully me and Michelle'll catch up with you before you disappear into your trench again.

Oh, and don't forget my Easter egg, will you? Mine's a Smarties one.

Fiona said...

Brian,
I did see that there are another two articles to follow!! :-)

I don't live in a trench - I live in a library!!! A very expensive one at that! Most recent addition human skeletal atlas for my human bone module - Very exciting!

My easter egg is a snickers!!

I'll see you very soon then!
F

Brian Hughes said...

Fiona,

A skeletal atlas? I could do with one of them, 'cos I haven't seen my feet beyond the mountain range of my stomach for several years now.

JahTeh said...

Thanks for that URL. The first time I put it in it took me to roach world but the second got me safely to the right place.
This is where I saw the article,
http://www.amherstbulletin.com/story/id/84479/
so you can see why I queried.

Brian Hughes said...

Witchy,

No problem.

I could, of course, argue that Bleasdale Circle in the Wyre is a druidical burial site...or a shamanistic one, at any rate...and that the body buried at Stanway sounds more like a well-to-do Romano/Celtic chieftain to me.

But academia needs to earn its keep somehow, and the odd bit of misleading, sensationalist nonsense justifies their massive expenses to the archaeologically ignorant British authorities.

Guess who got out of bed the wrong side this morning, eh?