Topographical survey

Topographical or contour survey is the recording of lumps and bumps in the ground! What may look like a confused jumble of earthworks at ground level can resolve itself into an understandable plan when recorded in a contour survey. Conversely, very slight earthworks that do not hold promise when seen from the ground can, when recorded, prove to be the remains of a very complex site.

Going the extra mile

Surveys are generally carried out either by building up measurements as a series of triangles with distances or angles measured from at least two fixed points (triangulation); or by measuring at right angles to a fixed baseline (offset survey). It’s always best to plot survey results whilst the measurements are being taken as it’s easier to correct any mistakes as you go along and you can check the drawing against the earthworks in the field. You will need garden canes and at least two long tapes and sheets for recording and drawing the results. An optical square will be useful for setting out the grid and doing the survey. For more complex surveying you could get hold of a dumpy level, theodolite or EDM with appropriate measuring staff .

Figure 1: Either triangulate by measuring distances between several fixed points and the monument or by measuring the angle from north.

Paced survey

This is the simplest form of survey and requires the least amount of equipment. Although it is possible to produce fairly good results, it won’t be as accurate as a measured survey and is probably best used for initial surveys. Ideally a baseline will be set out on the ground with a tape-measure and then earthworks can be plotted in relation to the baseline by counting the number of paces distant they are from fixed points along the baseline. It is possible to survey using a fence or a hedge as a baseline and establishing fixed points by simply pacing alongside it, but this will take longer than using a tape-measure.

Try to maintain a consistent length of stride (this is more difficult if you are tired or if the earthworks are on sloping ground!) and check your pace/stride length against the tape-measure, in order to estimate distances in meters. It’s best to check this for more than a single stride. For instance you may find that 11 paces = 10m, or that your normal stride length is 1m.

Offset survey

Offset survey also uses a baseline and measurements are taken at right angles to the baseline using a second tape measure. There are two methods of ensuring a right angle is made at the baseline: lay out a 3-4-5 triangle using a third tape-measure.

Figure 2: A 3-4-5 triangle to set up a right angle for an offset survey baseline

Or with the end of the second tape pegged or held securely on the point being surveyed and with the tape held taut, move the second tape backwards and forwards over the baseline to find the smallest measurement along the second tap-measure – the closest distance between the survey point and the baseline will always be at a right angle.

Figure 3: An offset survey, making sure the measurement from the monument to the baseline is at 90°

Alternatively you could use a compass, a cross head or an optical square (these are both small hand-held instruments which allow you to set out a straight line between 3 points and then to sight in another point at right angles to the line).

Prismatic compass survey

A prismatic compass has a flip-up sight that can be aligned on a survey point or a feature in the lanscape and the bearing from north read off the compass dial as an angle. By taking a series of readings from more than one point, it is possible to build up a network of bearings that can be plotted on a plan using a protractor. You will probably need to use a ranging rod or straight cane to mark the feature or point being surveyed as the prismatic compass must be held horizontally at eye level to allow the compass needle to move freely and to be able to read the angle.

Plane Table Survey

Plane table survey is rarely used these days as the equipment required is bulky and fiddly to set up.

Contour survey

A dumpy level, theodolite or EDM (electronic distance measurement) is used to take spot heights within a grid. The grid can be set out at 1 x 1m, 2 x 2m or sometimes up to 10 x 10m for very large areas. Strings or tapes should be stretched between the outside grid markers, whether these are canes or pegs. You will need one person to stand with the staff on the point to be measured and one to look through the surveying equipment and focus on the staff.

The dumpy level is always on a level horizontal plane and needs a measuring staff to take relative heights of the ground. The dumpy level must also get a backsight to a permanent benchmark in order to tie in the relative heights to the Ordnance Datum (mean height above sea level). The theodolite can move up and down as well as side to side and therefore only needs the tip of a ranging rod to focus on to take the relative height of the ground. A backsight should also be obtained. With both of these methods you will need to draw a grid that corresponds to the one you have set up and record the heights you measure on the machine at each equivalent point on your plan.

Figure 4: How to take a level

By making a backsight to a benchmark, you can work out the height above mean sea level (Ordnance Datum – OD) of the instrument. Then by taking the foresight, you can work out the height above OD of the archaeological earthwork or excavation. The formula is:

Benchmark + backsight = instrument height.

Instrument height – foresight = level above OD.

For a database of benchmarks across the UK go to www.bench-marks.org.uk.

The EDM (Electronic Distance Measurer) or total station can be tied in by locating it with prominent points marked on Ordnance Survey maps. They can then record the contour survey by focussing on a prism-topped staff. The prism allows the EDM to work out how far away the staff is as well as the height. The staff should, however, be exactly vertical and it takes some practising to get this right. You may want to use a plumb bob to help. There is no need to keep a grid plan and record the measurements as they are stored within the machine and can be downloaded on to a computer and transformed into a digital contour survey later.

Though all contour survey methods should have a grid, it is not necessary to ignore localised lumps and bumps and these can be measured too, as long as they are tied in to your grid by measuring the distance from each string or tape. It is easier to do this with the EDM as it automatically records the location of every measurement taken. In fact, the EDM does not need a grid to be set up and followed but it is good practice to ensure that enough measurements are taken to get a good result.

Topographical survey

Going the extra mile

Further reading