Wednesday, 17 July 2013

We love it when a 'plan' comes together...

Our volunteers putting the finishing touches to a scale plan

Over the past few days, if you've taken a look in our trench you'll have seen groups of volunteers drawing scale plans of the site, using the grid that we talked about in the last blog post. We've had a few interesting questions about the role of technical drawing in archaeology, so we'll try to answer them here!

Why don't you just take a photograph?

We do take photographs! Lots of them, in fact. The photographs form an essential part of the site archive. Every part of the site is photographed with a scale and a north arrow, and the details of the photograph (number of the feature, size of scale bar, related drawings) is noted in a register.
However, photographs can't tell the whole story. Perspective makes objects further away from the lens appear closer together, and whilst this can be corrected by digital rectification, this tends to result in a loss of quality. These problems can be overcome by using complex pieces of kit like the pole-mounted cameras used by Adam Stanford at Aerial-Cam, but....
...Another problem with photography is that it tends to 'flatten' features: it can be difficult to get a sense of depth and angle of slope through a photograph, and that information is crucial to interpreting archaeological sites.

So, why don't you just survey the site electronically?

Again, we do! It is standard practice on many archaeological excavations to survey the site using a 'Total Station' or GPS/GNSS staff-mounted survey units. This overcomes the distortion and 'flattening' effects of photography, and can produce an extremely accurate digital plan quickly and easily. However, there are details that we need to record that don't show up on an electronic plan. For example, one of the important functions of a plan is to record differences in the composition of different deposits. Is there variation in building materials? Flecks of charcoal in one area? A high proportion of stone in a layer? All of these details can be easily and quickly recorded in hand-drawn plans and sections (drawings of vertical faces).

Another reason for planning by hand (and I have to admit to being biased here, as planning is probably my favourite part of site work!), is that it really helps you to interpret the site, and forces you to take a really close look at what you're dealing with. Besides, many of the volunteers we're working with won't have access to expensive electronic survey equipment for their own archaeological projects, so it's important for them to learn the skills needed to do the whole process the old-fashioned way!

All our plans are drawn on 'drafting film', a translucent sheet printed with grid squares. Drawing is done with a hard pencil (usually 6H) at an agreed and consistent scale - in this case, 1:20 for plans and 1:10 for sections. One of the advantages of a translucent sheet is that drawings of different phases of the site as the layers are peeled away can be 'overlaid' to compare them.

So, next time you see us with drawing board and pencil in hand, you'll know what we're doing and why! Here's a photograph of one of the finished articles, all surveyed, drawn and levelled by our volunteers - Not bad for a first attempt!

Rob Hedge

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