Excavating an experimental catapult

We recently excavated the remains of an experimental machine designed to launch aircraft – which sadly was an idea that never fully took off. Designed as a prototype in the lead up to World War Two, the catapult didn’t work and the project was eventually abandoned. So what do archaeologists do with the remains of a huge concrete structure?

Well, apart from a bit of jet washing to clean the concrete, a lot of our techniques were the same! Not only did we uncover and clean the whole structure, but, importantly, we recorded every piece of evidence we found. This doesn’t only mean recording walls. Our archaeologists can’t learn everything just from looking at an object or building on its own. We need to know the whole context – where was every object found, what did it look like in the ground, what type of soil was it in?

We call this ‘preservation by record’. By recording every piece of evidence accurately and in minute detail, the information a site holds is preserved in an archive. Any future researcher can still analyse much of the same information that was available to us when we excavated.

Photos

Obviously, one main way of recording what is found is through photography. This can record the appearance of objects and features, and what they looked like when they were first excavated.

The accuracy of these photographs is crucial, so we make sure that the objects are clear and there is a scale in shot. This means everyone can be sure of the size of each object or feature just from the photo.

Our team don’t only use regular cameras, we also use drones to take aerial photos. This is especially important when recording a structure as large as the Harwell catapult. Aerial photos give a clear representation of the entire site at once.

During the excavation and recording of the Harwell catapult, thousands of photos were taken and saved as part of the catapult’s record. This recorded things like the impression of corrugated iron in some areas of the catapult walls, left behind by the mould that the concrete was poured into. This suggests that the catapult was built fairly quickly and using any materials to hand.

Geolocation

Our archaeologists also need to record the exact location of objects and structures so we can accurately map where everything was. We do this by using an extremely accurate GPS device and a piece of kit known as a total station.

While ordinary GPS that might be found in your car or phone will locate you to within a few metres, our GPS will locate objects to a few centimetres. Important features and objects will all have coordinates that pinpoint their location and help us build up a map of a site. It also means future researchers will be able to know where everything existed in the landscape.

Whilst something like this catapult, with its flat concrete floor and straight walls, looks easy to measure the depth of it is not quite that simple. What if the floor slightly slopes towards a drain? Do the arms that were designed to launch the planes slope upward?

Total stations calculate height and depth by measuring the angle and distance to a point from a location with known height. They find the angle and distance by bouncing lasers off reflective points.

One area of the catapult where the use of total stations was crucial was a small exhaust vent was found on the outside pit. It can be extremely dangerous for our archaeologists to excavate deep holes because there’s a risk of the sides collapsing over them.

Therefore, we had to excavate the vent to a certain depth, record everything and then take down the walls and start excavating again. By the time we reached the bottom, not much of the wall remained to measure. However, its total depth had been measured and recorded in stages by using the total station.

Interpretation

While photos can take accurate images and total stations and GPS can record accurate locations, they don’t tell the full story. Our archaeologists therefore write down what we’re doing and what we find in detailed reports. Here we can talk about the methods we used, the type of soil and how areas interact with each other.

This helps future researchers understand and interpret what is visible in the photography. For example, there are no photos of the complete exhaust vent as it had to be dismantled during excavation. We can explain why there are no images, and what we found as we worked on the area.

3D model

3D models are a great way to record features and structures, and we often use them to give people a chance to see the archaeology up close.

The Harwell catapult is a huge structure, and this model allows you to ‘stand’ in an archaeologist’s shoes. It can be difficult to appreciate the scale of the structure from photos, but a 3D model allows you to see the archaeology from all angles.

3D models need a combination of accurate location points and detailed photography to build an accurate picture. To achieve this we intensively photographed and measured the structure. We used thousands of images to build the model, which is now free for the public to access online.