Sandsfoot Castle; metric & photographic survey

Sandsfoot Castle is a Henrican Castle built during the 16th Century on a cliff overlooking Portland Harbour, Dorset. Together with Portland Castle, it provided defence of the harbour and surrounding roads, protecting shipping and preventing invasion. It is a Grade II* Listed Building and a Scheduled Ancient Monument.
 
Wessex Archaeology were commissioned by Weymouth and Portland Borough Council to undertake recording and survey work to contribute to a Conservation Management Plan as part of a Heritage Lottery Fund bid to improve access to the ruinous remains of Sandsfoot Castle. After consultation with the client, a combination of techniques were deployed including terrestrial laser scanning (TLS), rectified photography and panoramic photography.

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Producing a 3D record

As with most TLS projects, the fieldwork was undertaken in a very short space of time, with photographic survey work being undertaken simultaneously. This was particularly important as the interior of the castle is a hazardous place with the potential for falling masonry. Laser scanning was undertaken using the latest Leica C10 instrument capable of recording up to 50,000 3D measurements per second. In addition to allowing the surfaces of the castle to be recorded to around 2mm precision, the C10 captures full colour information resulting in a coloured point cloud.
 
Survey control for the TLS component was implemented using a closed loop traverse with Total Station Theodolite (TST) instruments, a commonly used surveying technique to ensure unavoidable error introduced by the tolerances of the survey instruments around the survey control network is minimised and quantified. The survey control network was then related to the British National Grid coordinate system using Global Navigation Satellite System (GNSS) to ensure that every point in the TLS data is within the standard Ordnance Survey coordinate system.
 
The TLS data was then used to produce a series of orthographic images and was also used to produce elevation drawings of each elevation of the castle. Stone by stone elevation drawings were required and this was facilitated by having a high resolution, coloured point cloud to work with.
 
It was also necessary to understand the nature of the levels inside the castle and immediately around the castle to help with any design proposals to enhance visitor access. To facilitate this, the TLS data was imported into a Geographic Information System (GIS) where a Digital Surface Model (DSM) of the ground surface could be produced. This DSM was then used in conjunction with the photographic record to interpret the observed remains inside the castle and to provide accurate height measurements across the ground surface and on the remaining archaeological deposits. The GIS was then used to produce illustrative plans of the interpretations and height data as a grid of levels plus contour lines.
 

A complementary photographic record

In addition to the TLS component of the project, rectified photography was used to produce a visual record of each elevation, both the internal and external elevations. This was particularly challenging for the seaward side as the only available vantage point to take photographs from was the beach at low tide, resulting in photographs taken looking up at the castle atop the cliff. Survey control for this element of the project was provided using Total Station Theodolite (TST) instruments.

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Panoramic photography was also used to create interactive bubbleworlds suitable not only to view and investigate the castle and its surroundings as part of the overall record, but with a view to using them to create online resources for the public to interact with and find out more about the castle. Each panorama involved capturing a series of photographs from a single location with an equal angular separation between them. A special tripod head was used to ensure the camera rotated around the focal point of the camera, reducing distortion to a minimum. These photographs were then stitched together using specialist software to produce the final product and an index provided as a Keyhole Mark-up Language (KML) file for use in Google Earth or Google Maps or any other GIS platform.
 

Conclusions

TLS is an ideal technique for rapidly surveying potentially dangerous structures such as ruins due to the reduced time spent on site. The TLS data, when captured in colour, provides a good visual record as well as being highly metrically accurate.
 
Photographic techniques, particularly panoramic photography provides an excellent user experience for the visualisation of heritage sites and furthermore can be used as the basis for engaging outreach materials aimed at the general public.
 

Acknowledgements