These stages can be divided into three:
• Collect together the relevant photographs and select those which best show the archaeological information and can be matched to a map;
• Transform those photographs to match the map;
• Prepare an interpretative drawing in GIS or CAD.
Selection of photographs
Whenever possible, all photographs of a site or small area should be assessed for fitness of purpose. We need photographs that show two things – a site in good detail and sufficient control points to allow them to be accurately located on a map or other scaled background. In order to map a site, a photograph with good control points takes priority over one showing good archaeological details but which has insufficient control enable it to be tied to a map. However, there are ways in which these can be added at a later stage.
A good spread of control points on a photograph is vital to establish the exact location or size of features. A control point is an object, such as a junction of field boundaries or corners of buildings, which can be identified on an aerial photograph and also on the map or other georeferenced background. Control points should always be fixed at ground level – i.e. ground control points (GCP). Using several GCPs (most computer methods require at least four) a photograph can be transformed (i.e. accurately positioned and scaled) to match a background. Once this has been done interpretation and mapping can progress.
Computer transformation is best done using programs that have been written to deal with oblique photographs – for example, AERIAL and AirPhoto. Although most GIS can georeference photographs, the algorithms they use are more suited to vertical photographs and satellite images than to oblique photographs. At their most basic level, these programs, allow a map and aerial photograph to be combined. Pairs of matching control points are added to each and a transformation routine then scales and locates the photograph on the map. This can be done with as many photographs as are required to cover a chosen site or area remembering that photographs taken on different dates and under different conditions may show complimentary information. Once transformed, the images are 'true to plan' and can be used as a basis for accurate mapping, in GIS, CAD or other drawing tools.