Small Unmanned Aircraft and archaeology – why now?

In 2015 the Castle Studies Trust and the Morecambe Bay Partnership worked together to record the remains of Gleaston Castle in Cumbria. By using an unmanned drone to photograph the castle we were able to reach every part of the building. Below is an excellent explanation from Museum of London Archaeology’s Dr. Peter Rauxloh of how this technique works. It was originally published on MOLA’s blog.

Archaeologists have known about the benefits of investigating archaeology from an aerial perspective for more than 150 years, arguably longer, but the day-to-day use of aerial survey in MOLA’s work has been hampered by cost.

The model drone was a very popular Christmas present last year; a clear indication that cheap aerial photography has arrived. The big innovation from the perspective of archaeological aerial survey is the arrival of affordable digital photogrammetric software and automated flight control.

Photogrammetry is the practice of taking measurements from images. It is based on the fact that the position of an unknown point in 3D can be calculated if it is viewed from at least two other known points. This is the principle of triangulation, as important to map makers in the 17th century, as it is today.

The software in question uses the principle of photogrammetry and applies it to the processing of digital images to create 2D orthomosaics and 3D models. In essence, the traditional technique of aerial photogrammetry used a small number of high quality images taken with expensive specialist cameras from real aircraft, on which a small number of common points were identified between adjacent images. The new method uses a large number of images, taken with a standard consumer grade digital camera from cheap aerial platforms and is able to identify thousands of common points between images. Thus the technique tightly locks together overlapping areas to create robust orthogonal mosaic and highly detailed 3D models.

The amount of overlap required between images in order to create robust models is very high, typically 75-85% between each successive image. This means that 85% of one image must still be visible in the next image, but how does one ensure that critical degree of overlap is maintained?

Typically an aerial survey will be flown in a grid, much like a tractor ploughing a field, and each successive image needs 75-85% overl ap with its predecessor. Moreover once it has turned and starts to fly back over the site, it must also get 60-70% overlap between the images on each leg. For small survey areas the aircraft is flown manually since it is easy to ensure the area is well covered with images,  but it is far more difficult to maintain the required consistency on a large site.  This is why MOLA use an autopilot and have it follow a flight plan.

A flight plan is a group of 3D coordinates or waypoints between which the aircraft navigates using its on-board GPS. To position these waypoints so the images overlap a number of variables need to be balanced. These include the camera’s field of view, frequency of image capture and the desired flight height and the speed of the aircraft. These in turn need to be considered against the size of the area to be flown and how long the aircraft can safely stay in the air.

Below you can fly round the model of Gleaston Castle.

Gleaston Castle
by aerial-cam
on Sketchfab

Bringing Pleshey Castle’s history to the public

Betrayal, intrigue, fire, a king arresting one of his men, and the richest man in England. In 850 years Pleshey Castle has seen it all.

The village and castle owe their existence to Geoffrey de Mandeville who founded both in the 12th century. At the height of his power he was the richest man in England apart from the king, but lost it all when he was accused of being a traitor. Pleshey was owned by his descendants (though in the hands of the king) before it passed to the Bohun family by marriage. It remained with them until 1380 and in 1419 Pleshey Castle became part of the Duchy of Lancaster. Because of the Duchy’s ownership we have detailed accounts of building work in the 15th century. By 1559 the castle stood empty, and now survives as some very impressive earthworks.

Throughout its history the castle was remodelled several times – sometimes to improve the accommodation, at least once to repair fire damage, and once because it had been partly demolished. Excavations between 1972 and 1981 led by S.R. Bassett tried to shed light on this complicated history. Unfortunately this important work never saw the light of day. Notebooks and context sheets were left in the archives, out of reach of all but the most determined until now.

Our tour group in the outer bailey
Our tour group in the outer bailey

Patrick Allen and Nick Wickenden are leading the efforts to get the results of the excavation published. The Castle Studies Trust have funded the creation of detailed drawings showing the work. They provide an invaluable visual reference, and show how parts of the castle have developed.

Earlier this month we visited the castle with some of our donors. As well as exploring the castle it was a chance for donors to talk to trustees and see first-hand how valuable the charity’s work is.

Crossing the moat surrounding the castle you get an idea of how impressive the site is. The motte at the heart of Pleshey is an impressive 17m tall while the ramparts are nearly 5m high and cut you off from the outside world. You can still see where the chapel was excavated and a section was cut through the rampart.

Pleshey is privately owned so if you want to visit you have to arrange it in advance. This is typically of the sites we work with, and we give donors the opportunity to look round. Patrick and Nick gave us a tour, explaining the colourful history of the site and what they were doing to publish the excavations. Behind the scenes, the illustrations complemented the research and helped understand the phasing of the site.

It can take years for excavations to be fully processed and reach the stage where the results are ready for the general public. Sometimes that stage is never reached making it much more difficult for people to access information. Because of your donations we’re bridging the gap.

We’re looking forward to seeing the final results!

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4 Castles to Visit this Month

Every year historic buildings across England, Northern Ireland, Scotland, and Wales open to the public for free. It is a great excuse to go out and visit some castles. You can find out more details online. Here are four places we find interesting.

Mugdock

Mugdock Castle in Scotland. Photo by Ryan Woolies, CC-BY-NC-ND 2.0.
Mugdock Castle in Scotland. Photo by Ryan Woolies, CC-BY-NC-ND 2.0.

A nine-mile drive from Glasgow, Mugdock Castle was built in the 14th century and converted into a mansion about 500 years later. The Grahams who lived there until the 20th century were an influential Scottish family. Visitors can walk round the gardens which were laid out in the Victorian period and the ruins of the castle.
Mugdock Castle is open from 10am to 4pm on Saturday 12th and Sunday 13th September.

Carrickfergus

Carrickfergus Castle, Northern Ireland. Photo by Andrew McCoubrey, CC-BY-NC-ND 2.0.
Carrickfergus Castle, Northern Ireland. Photo by Andrew McCoubrey, CC-BY-NC-ND 2.0.

John de Courcy began building Carrickfergus Castle on the coast in 1177 and more than 800 years on it is one of the best preserved Norman castles in the country. When looking round, try to imagine the gatehouse being twice as tall. It was shortened in the 16th century when the castle was adapted to use cannon.
Carrickfergus Castle is open from 10am to 5pm on Saturday 12th and Sunday 13th September.

Dolbadarn

Dolbadarn Castle by Sian Monument. CC-BY-NC-ND 2.0.
Dolbadarn Castle, Wales. Photo by Sian Monument, CC-BY-NC-ND 2.0.

Dolbadarn Castle was built by Llywelyn the Great, Prince of Gwynedd, in the 13th century. The round tower is typical of Welsh design. Its close link to the Welsh princes meant that when Edward I invaded and built his own castles, timber from Dolbadarn was used to build the castles as Llywelyn’s castle was partially demolished.
Dolbadarn Castle is open from 11am to 4pm on Sunday 6th September with tours every hour.

Guildford

Guildford Castle, England by tps58. CC-BY-NC-ND 2.0.
Guildford Castle, England. Photo by tps58, CC-BY-NC-ND 2.0.

At Guildford Castle you have a royal palace, a great tower possibly built by King Stephen in the 12th century, and gardens. You can climb to the top of tower which sits on top of a mound. There’s a chapel inside with old graffiti. On the outside you can see where an extra floor was built on top of the tower.
The castle is open from 10am to 5pm on Saturday 12th September.