"...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 RCAHMS 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.
The true colour image above is an orthographic 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 is compared to topography derived from the same data but with the colour removed and lit from a series of virtual 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.
These kite aerial photographs, taken on September 19th 2011, were among the sequences which were used to generate the orthographic images above.
Interactive Site Viewer v.1
Bridge of Cally Hut Circles
The settings above effect this orthographic (plan) view of an obscured hut circle site at Bridge of Cally in Perthshire, Scotland. Use the slider to compare a true colour orthophoto with topography described by simulated low angle light. Dragging on the image makes a measurement.