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2.2 Acquisition procedure

Of great importance for any archaeological use of remote sensing images is their appropriate georeferencing or rectification. Only in this way is it possible to relate them to archaeological features on the ground and to combine them with other data in a GIS, for example with geophysical results. Satellite images are increasingly delivered in a georeferenced format that clearly specifies their location. Since most of them were taken vertically, distortions due to changes in topography are considered to be small so that a simple affine transformation is deemed sufficient. However, only the position directly underneath a sensor is recorded vertically and areas away from this position may be distorted by the undulating landscape. There is therefore a market for photogrammetrically rectified remote sensing data based on accurate topographical maps, so called ortho-photographs. Software to produce such results is available and it can be expected that data will increasingly be delivered with these corrections applied.

The situation is more complicated for aerial photographs, which in many instances are taken from an oblique angle from a small aircraft. With the reduced costs of in-flight GPS equipment it has become possible to assign well-defined coordinates to each photograph taken. However, these coordinates only refer to the camera position; the targeted archaeological feature may lie somewhere in the area around it. To provide more accurate information for the target location, various solutions can be suggested. Due to the continuous recording of the aircraft's GPS coordinates, its orientation can also be determined (e.g. 'flying due north'). If some information on the relative orientation between camera and aircraft is available, the position of the photographed target can hence be estimated. Such information can either be a vague verbal description (e.g. 'through right window, looking straight ahead') or a mechanical link between the two systems that measures two angles accurately. Alternatively, it may be possible to obtain an absolute record of the camera direction using an attached digital compass. Together with the aircraft's GPS coordinates the target location can then be calculated.

The continuous recording of flight-path coordinates is also an essential aid in large-scale recording campaigns, like the English Heritage funded National Mapping Programme (Bewley 2001). The incorporation of such information into a GIS allows the allocation of flying resources to the least investigated areas.

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