'What's that? The field of ripe barley at two o'clock, two miles, near the L-shaped wood? Those patterns can't be natural ... too regular. Let's have a closer look ... There! A series of rectangles one with a circle inside. We'll have that. ... Opening window! Wing down a bit! ... more... more... that's great!'
Grab the 35mm camera - photograph the target - reach behind you for the 70mm camera - wait - it's clearer from this angle, photograph it again. Log your position on the GPS, mark up the report form on the knee pad and go back to scanning, as you search for a new target.
All this is what aerial archaeology, or at least the photographic reconnaissance part of it, feels like. Many people are now familiar with the concept of remote sensing - epitomised for most by the geophysical survey. Well, this is remote sensing too and, lets face it, you can't get much more remote than 2,000 feet above ground level, travelling at 90 miles an hour, constantly flicking your eyes between the map in your hand and the landscape below.
Aerial archaeology sounds - and is - exciting, but there isn't much glamour about it. The inside of a light aircraft is not roomy, especially a two-seater. After three and a half hours you are aching, tired, and in winter pretty cold, but the payback for this discomfort makes it all worthwhile. Most archaeologists do what they do because they relish the thrill of discovery, and that is one thing that aerial archaeologists get more of than most.
To see below you the unmistakable shape of an unknown Roman camp, emerging in outline under rows of sugar beet, or to pick out subtle earthworks highlighted by the play of light and shadow of the low winter sun, can be an unforgettable experience. For now, though, let's get back down to earth, and consider what it is the airborne archaeologist is looking for.
An image of Cawthorn Camps is compiled by mapping
How then, is the archaeology mapped from all this photography? All photographs, obliques and verticals, are digitally rectified to remove inherent distortions, due mainly to height and camera tilt, and transformed into corrected plan views.
The accuracy of the resultant photo-map is largely dependent on the accuracy of the control that the photo is rectified against, usually Ordnance Survey maps and height data, at 1:10,000 or 1:2,500 scale. For the highest possible accuracy, new vertical photography of the site is taken using a calibrated camera, and control is surveyed on the ground.
A computer software programme automatically generates height information, by comparing adjacent photo frames (remember that overlap?), and a composite orthophoto (a photo-map accurate to a few centimetres) can then be produced to the desired scale (see the image of Cawthorn camp). This is made possible through the use of a device called a stereoscope.
Stereoscopy involves two photographs, vertical or oblique, taken in close succession, each frame capturing a slightly different view of the same subject. They can be viewed stereoscopically, using no props, to recreate in the mind's eye a 3-d model of the landscape - or you can use a stereoscope to make it easier. The Victorians had stereoscopes in their parlours and bought views of famous places to marvel at, and the 'magic eye' craze of a few years ago worked on the same principle.
A stereoscope, at its simplest, is a pair of lenses mounted on a folding stand and costing less than £30. The device helps make it easier to see earthwork archaeology on vertical and oblique photographs, as things appear to 'stand up' out of the photo. Anyone interested in this process can take their own 'stereo pairs' of photographs on the ground. After taking one photo, you just take a couple of steps to the side and take a second photo. Try it next time you are out with a camera. The only tricky part is learning how to view them if you don't have a simple stereoscope, but it isn't impossible.