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 draughts 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 improvising 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.