Photo-rectification

Our site at Horse and Groom is now over, but we still have plenty of work to do. One job is to create elevations of all the walls of the medieval buildings. These will be used to illustrate the final report, but most importantly they form a key part of our record of the site. We are using a technique called photo-rectification as it is a quick and effective method that takes less time on site, but still allows for accurate results.

Photo-rectification is a well established archaeological method that can be used to produce scaled drawings from photographs and it is widely used to record excavated and standing buildings. The technique is particularly effective on structures whose external surfaces are composed of a series of flat surfaces (such as walls), rather than complex 3-D structures such as vaults. Photo-rectification can be used to produce both elevations and building plans and is a relatively fast technique on-site, although the processing takes additional time in the office.

Lee fixing targets to the walls using a glue-gun

Reflective targets (Ground Control Points) are fixed to the structure and the precise location of each target is recorded in 3-D using a Total Station/reflectorless EDM. These target positions are imported into a CAD or GIS program where they create a 3-D scaled frame to which the photographic images will be calibrated.

A series of photographs are taken of the structure -making sure that several targets are in each image and that the images are taken angled square on to the structure in order to reduce image distortion. Overlapping images are taken to further reduce image distortion, and an overcast day is best as this eliminates shadows in the images. A record sheet is filled out for each photo-rectification job, listing the photo and target numbers, and a sketch of the elevation showing target locations and context boundaries. These record sheets are essential as they give the person processing the jobs all the information in one place.

Where necessary the photographs are ‘stitched together’ to create a single image and are then imported into the CAD or GIS program where they are stretched or ‘rubber-sheeted’ to fit the targets. This creates a scaleable image which can then be manipulated to allow scale elevations and plans to be made by tracing the outline of the walls and individual stones.

Left to right: the photographs of each elevation are rectified, then the outline of each stone and context traced off in CorelDraw, the final drawing can be annotated and laid out to the required format for archive or illustration

For our site rectified versions of the AerialCam overhead photos will be used to add stone-by-stone detail to the plans of the medieval buildings, whilst rectified side-on photographs of each wall will allow detailed stone-by-stone elevation drawings to be created. Additional hand-drawn elevations and plans were taken as required to fill in any gaps in the photo-rectification record.

The process does take time but it’s a lot quicker than drawing all the elevations by hand at scale, and of course it can be done in the warm and dry!