Aerial Photography and Visualisation for Built Heritage - PhD Portfolio by Kieran Baxter
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Tuesday, 22 November 2011

Hut Circle Topography at Bridge of Cally

"...traces of the others [hut circles] are very slight and were only recorded by oblique sunlight at about 1pm on a winters afternoon..." (RCAHMS, 1987)
This description prefaces the site record of Rochallie at Bridge of Cally which features two groups of hut circles among cairns and other features. The practice of waiting for snow and low light in order to see the topography of an un-excavated site is still widespread. With this in mind I set out to replicate the effect described at Rochallie using KAP and photogrammetry.

This first image is an orthographic colour projection derived from two KAP sequences. It is annotated with my interpretation of the hut circles and features based on the RCAHMS report and my own observation.

This second image is derived from the same data but with the colour removed and lit from a series of low angle lights. The three main hut circles can be seen in the center along with the other features. The photographs below give an idea of how difficult it is to perceive the topography in normal conditions, the more distinct hut circles and cairns are visible only on close inspection.

Hopefully these images demonstrate a fairly successful test of photogrammtery as a method for recording an obscured site. With hindsight I would attempt to capture the entire area in one pass for better coverage rather than going for multiple angles of such a low lying site. Although I was concerned that the software (123D Catch) would have issues with the similar grass texture this was no problem. Instead the main issue was recognizing the continuity from one frame to the next, something to consider when capturing in future.

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