A survey technique known as photogrammetry has been used at the amphitheatre to help us understand the walls that you can see on the site.
This type of survey involves making accurate measurements and drawings from photographs. Rather than using just one photograph, two overlapping images taken from slightly different positions are used. Commonly known as 'stereo-pairs' or 'stereo-photographs', these two slightly different images can be viewed in 3D in a similar way to how most of us see the world - our left eye is seeing one image and the right eye another, albeit slightly different, which our brain is constantly combining together to form a 3D image of whatever we are looking at. By making a camera 'the eyes' and a computer 'the brain', it is possible to computerise the process to accurately survey and measure any object we see.
[Above] The surveyor with a blanket on his head is using a survey camera to take the stereo photographs of the walls (this is to keep the light out – not keep the rain off!). The other surveyor is using an electronic theodolite to survey the targets on the walls.
At Chester the technique is being used to create detailed drawings of the stone walls that are visible within the excavated remains of the amphitheatre. Stereo-photographs of the walls have been taken using a special camera. Such cameras provide excellent images ideal for survey purposes. To enable measurements to be taken, a scale is introduced into each photograph using small black and white 'butterfly-style' targets which are temporarily stuck to the wall. Observations are then taken to the centre of each using a theodolite - a device for accurately recording angles and distances. When combined, these measurements accurately locate each target across the site and allow very accurate drawings to be made from the photographs.
[Above] Stereo-pair of photographs of part of the remaining walls at Chester's Amphitheatre
Drawings are produced using a computerised machine known as a photogrammetric plotting machine or 'stereo-plotter'. As well as allowing the photographs to be viewed in 3D, these devices allow lines to be accurately traced over the individual stones to form the required scale drawings.
These drawings are used by archaeologists to work out which parts of the remaining walls are original and which have been replaced. Comparing the different types of stonework should help us find out if the amphitheatre was rebuilt or re-used in any way after the Romans left.
The computerised data can be used to help create a virtual reality model of the site.
This first stage of this survey is now complete. A full photographic record of the exposed amphitheatre arena stonework is now complete and measured elevations have been drawn electronically. These computer-based drawings will be used by a stone specialist to identify and analyse the various phases of building and re-building.
[Above] Sample of computer drawn elevations
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