While we mostly used triangulation to delineate interesting rocks on our drawing, we found that the results worsened as we approached the edge of the trench, as the inaccuracies of the method became a high percentage of the measurement taken. Therefore, for the rocks around the centre of the trench, we decided to use offsetting and ‘swinging the tape’ in order to record these rocks.

Having pegged a tape horizontally along an edge of the trench, a second tape was used to measure horizontal displacement from this edge. In order to do this accurately, an angle of 90 degrees must be obtained between the two tapes. Rather than measure with a compass or optical square, we opted for the quicker method of holding the end of the tape firmly on the rock corner and then adjusting the tape so that it read the lowest reading possible, as the lowest reading would be obtained when there was a 90 degree angle between the rock-point and the baseline tape.

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Written on August 8th, 2012 , Diploma Year One, Techniques

Whilst this technique obviously has many applications, it was encountered in the process of recording significate rocks within a trench otherwise full of rocks so is written to demonstrate this scenario.

You will need two surveyors tapes, preferably of different colours as that makes it much easier to record and minimises errors (proven by experimentation!). Secure each tape at a corner of the trench. At the start of the recording, we had a white tape and a yellow tape, so labelled the trench corners as W (for white) and Y (for yellow).

Taking care to keep the tapes horizontal (not parallel to the ground as we were on a crazy slope) bring the loose end of the tapes together at a corner of a rock. Note the readings on both tapes. Take readings at all interesting points of the perimeter of the rock, as these will describe the outline when you come to draw it.

Nicky demonstrating the multi-colour tape technique. July 2012. Copyright K Bragg

Once your rock has been described as a series of Y and W readings, it is time to draw! Drawing as soon as possible enables you to see if it has come out at all reasonably-shaped. Remember here that you are interpreting where the edge of a rock is, and the scale at which it is to be drawn will not allow much accuracy of depiction, so even when you do this carefully it is perfectly possible to end up with something preposterous.

You will need to have prepared your drawing with a scale outline of the trench (in our case we used 1:20 as a scale) with your Y and W points (or whatever you want to call them) marked on the drawing. You will need a pair of compasses with which to mark out the distance from Y or W – put the point of the compass against the scale rule and measure out the appropriate reading. This gives you the distance scaled to match the drawing. If you dislike using the scale rule constantly, it is much easier to draw out the scale as a reference on your drawing. We drew the 1, 2 and 3 meter points, and then another metre with finer subdivisions so that you can put the point of the compass at the correct number of metres away and then adjust for the sub-metre fraction. Which is harder to explain than to do. If you want 2.24 metres, you put the point of the compass at the 2 mark and then add on the 0.24. This avoids the need to mark in graduations for every metre, and stops you wearing a hole in the zero point!

Scale for drawing.

Using the compasses, draw arcs of the appropriate length from the appropriate corners and where they cross is the point on the rock that you measured (hopefully!). Repeat for all the points you recorded and then freehand draw the appropriate outline. For the more interesting rock shapes this was aided by a sneaky vertical photo of said rock.

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Written on August 3rd, 2012 , Diploma Year One, Techniques

Today started with half an hour spent backfilling the site of last year’s excavation: light-coloured limestone was moved from the spoilheap to the excavation (the light colour denoting lack of exposure and therefore was from the excavation, whereas the darker limestone was a clearance cairn) and then a covering of soil tipped on top. As this was quite tiring, there was a break immediately afterwoods so we could recover!

Site Reconstitution: backfilling last year’s excavation. July 2012. Copyright K Bragg.

As I’d turned up a week after the start of the dig, the trenches were already open and being cleaned, so my first task (and one that was to take several days to accomplish) was to produce a drawing of significant rocks within a trench. The reason for this is that we are looking at potential stone-built structures and amidst the mass of visible stones, we wish to pick out any that could be meaningful.

First significantly-large rocks plotted. July 2012. Copyright K Bragg

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Written on July 15th, 2012 , Diploma Year One, Excavation Notebook, Fieldwork

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:

Because it takes very little to persuade me to go clomp round the Vale of Pewsey, I decided to join the Wiltshire Archaeological and Natural History Society’s Archaeology Field Group as they did a spot of fieldwalking in Stanton St Bernard.

The weather was cold, damp and misty, and there had been quite considerable rain in the recent past, judging by the state of the land and the depth of the puddles. Not ideal weather for looking for things.

There were a couple of other factors that rendered me even less useful than might have been: I should wear glasses, and seldom remember this fact, and I currently have a sinus infection, rendering me febrile and exhausted. But I was determined to attend and gain some fieldwalking experience.

I’d been on a ‘finds day’ with the¬†renowned¬†linear-obsessive, Paul Tubb, and could probably tell pottery from stone now. But that was processing things found in bags. Far easier than spotting them on the ground. And warmer, drier, and by far less tiring.

Still, we spent the first hour laying out the grid, which mostly involved doing sums and fighting the wind for custody of the tape. By the end of that hour I was fairly tired after trundling back and forth across a muddy clay-ey field. I’ve always wanted to be a bit taller, but not by the addition of clay to the soles of my feet.

Laying out the fieldwalking grid

Laying out the fieldwalking grid

That picture doesn’t even begin to describe how muddy it was.

Still, having set out rectangles 50m long and 20 wide, we proceeded to trundle around inside them for 10 minutes picking up stuff and staring at it. It’s quite hard to see stuff that’s covered in sticky mud (especially without glasses). We were supposed to be looking for worked flint, something that I’m quite fond of and attracted to without thinking about it. But I didn’t find any. A few oyster shells, some mediaeval pot sherds, a bit of glazed tile and some post mediaeval bits of pot were about all I managed to see.

I only lasted a few hours before starting to feel really quite unwell, so I scooted off when they stopped for lunch. But it was worth doing.

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Written on November 20th, 2010 , Fieldwork Tags: , ,

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