Digital Surveying Instruments

3. Digital Photography and Photogrammetry

Digital photography and photogrammetry is now sufficiently advanced that it can meet even professional requirements. It's place on archaeological excavations is ensured by the ever sinking price of equipment. Direct advantages lie not only in the manifold potential uses in archiving, but also in the fact that digital images can be transferred directly onto computer to monitor their quality. Poor quality images can be discarded and then re-recorded. This avoids the time delay between recording and development of slides and other copies which was seen as the main disadvantage of this method up to now. It is especially advantageous in archaeology when images must be promptly processed, for example, if a grave find must be lifted on the same day. EDP also plays a decidedly important role in the post-processing of images with such functions as computer supported image rectification, colour matching and conversion of excavation photography into plans. I shall, however, restrict myself here to simple photogrammetry with image rectification, which could, of course, be replaced or supplemented by many other more expensive methods.

When documenting excavations, care should be taken to ensure that any photographs are shadow-free. A gantry or a ladder with independently adjustable legs is useful for taking photographs of feature quadrants from as vertical an angle as possible. By avoiding skewed or slanted photographs, the occurrence of the infamous parallax error can be kept to a minimum. For image rectification, at least four photogrammetry control points, set out and surveyed with a total station, are required for every quadrant. Slides can then be transferred onto computer using a slide scanner and digital photographs can be transferred using a computer interface. We use software solutions produced by AutoDesk for rectifying and assembling photogrammetry. AutoDesk also produce AutoCAD which is the platform for our system solution ArchaeoCAD. The AutoCAD application, CAD Overlay 14, allows trouble-free rectification and assembly of photogrammetry images and rapid compilation of two dimensional feature plans for plana and sections. Using CAD programs which support hybrid raster and vector processing, these photo plans can then be laid out again in the background, re-digitised directly on screen and enhanced by the addition of supplementary information (Fig. 7).

Fig. 7 Kuenzing, near Deggendorf, Lower Bavaria. Screenshot of photogrammetry documentation of a Roman horse grave created using ArchaeoCAD and CAD Overlay

Black and white or even colour printouts of the rectified plan quadrants are available whilst still on site. Laser printouts should be used where possible as they are more weather-resistant. These scaled printouts are used as a basis for the addition of supplementary documentation details, finds management data or further observations. This data can be entered directly into the printout or recorded on transparencies. When using this simple method of image rectification, which is not equipped for three-dimensional photogrammetry (there are other, more powerful programs available for this), the planum in question should not contain any significant differences in elevation. In order to acquire the three-dimensional data, the plan quadrant must be levelled using a total station. This method of documentation greatly accelerates and streamlines the working process, especially when many graves must be documented within a short space of time. The division of labour on an excavation should be thoroughly organised as a single screen can be constantly tied up even when only used for the necessary image rectifications.

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