Stokeleigh Camp, Leigh Woods, Bristol

Grid Reference: ST 5595 7328

Overview of Activities

The site selected for the survey was an Iron Age hillfort in Leigh Woods: Stokeleigh Camp (ST558733). Although the ramparts have mostly been cleared of tree cover, the site is still surrounded by trees, which made finding a suitable benchmark, to tie it to the national grid, tricky. Therefore a divorced survey was proposed and meant that we could concentrate on the surveying techniques within our own site grid without worrying about the exact location. The site grid was set in such a way that the origin was 1000, 1000 (to avoid the need for negative numbers).

We had already prepared the drawing boards with centimetre grid paper and a layer of Permatrace over the top.

We started by performing a reconnaissance of the site: walking the length of the ramparts to judge which parts would be suitable for the surveying exercise. We were slightly limited in our choice due to the equipment, as to record the rampart via offsetting techniques would have required us to be able to hold the tape level at the highest point of the bank, and that would not have been possible in some places due to the height of the bank.

 

Bank with ranging rods sticking out of it

Part of area selected for survey (a nice low bank). Copyright K Bragg 2011.

We therefore selected an area at the modern entrance to the site as having both interesting lumps and bumps to look at, and also a slightly lower bank that could be reached with a horizontal tape held at the top of a ranging pole.

The next step was to choose a point to act as the master control point for the survey. The criteria for the location for this were that it would afford visibility to all the points we wished to include. We selected a knoll in the centre of the study area as giving a good view of the earthworks and put up a ranging pole as point A. It should be noted that I used my own convention for recording points, which is simply to start at A and work through the alphabet. My first point is the master control point, by convention.

Point B was chosen at the base of the main (inner) rampart of the site and the distance from point A to point B was recorded by extending a fibreglass surveyors tape perpendicular (i.e. perfectly horizontally as we need a plan view and therefore need to ignore the contours of the ground) to the poles and measuring the horizontal distance between them. Point C was then added and then the distance from C–>B and A–>C recorded in the same way. These three points were then drawn onto the site grid at 1:500 using a scale ruler to work out how far each point was from the other point and therefore plot the intersection of the two distances to give the third point (which would have been easier with a pair of compasses). Further points were added, up to point H, and drawn onto the plan.

This framework of points formed the skeleton of the plan drawing of the site, and the lines between the points could then be used as a basis for offsetting. This involved setting out the tape exactly perpendicular to the pole at either end (sometimes involving holding the tape half-way up the pole when the ground surface was not flat) and then traversing down it at intervals, taking measurements at 90 degrees to the baseline tape to record slopes and other features as a distance from the baseline. Each time there was a break in slope, or something interesting to record, a distance reading was taken, and a note made of what the recording was of e.g. change from gentle slope to steeper slope. In this way we know at what distance from our baseline the slope changes, and therefore when constructing a hachure plan, we know at what point to change the weight of the hachures.

Armed with this information it is then possible to update our framework plan with the actual shape of the earthworks (in the horizontal dimension, we did not take levels to determine heights, so the heights indicated by the hachures will be approximate and based solely on a guess at relative slope angle). This work is yet to be completed.

Once we had understood the principles involved in setting out control points using tapes, the Total Station was set up and used to plot the locations of a set of new control points. This involved pointing the Total Station at a prism on a pole, and writing down the reading obtained by the device bouncing laser beams off the prism and measuring the response time. The Total Station had been configured with an arbitrary North point, to make plotting the points easier. This can be translated to magnetic North by calculating the difference in angle. The control points we set out with the Total Station were all designed to allow us to tackle the inner bank of the ramparts (we had previously concentrated on what looked to be a building just inside the ramparts). Using the same offsetting technique, we wrote down the distances at which the slope changed character in order to draw it at the next session.

 

total station

Paul setting up the Total Station. Copyright K Bragg 2011

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Written on May 8th, 2011 , Certificate Year One, Field Trip, Fieldwork Tags:

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