Comparative assessment of analytical and digital photogrammetric methods in the construction of DEMs of geomorphological forms

Traditionally, landforms have been measured using a combination of field survey techniques and analogue and analytical photogrammetry. With advances in computing power digital photogrammetric solutions are now seen as offering an affordable and cost effective way of mapping topographic features. This paper looks at the relative merits of each of these photogrammetric techniques, in terms of the practical considerations involved in data extraction and accuracies that can be achieved for a case study. The site investigated is Maiden Castle, situated southwest of Dorchester in Dorset, UK. This paper examines the relative accuracies of both digital and analytical photogrammetric techniques in relation to ground control points recorded using a global positioning system (GPS). Colour stereoscopic aerial photographs were taken of Maiden Castle, Dorset, in November 1999 at a scale of approximately 1:4000. Stereo models were constructed using pre-marked ground control points positioned using post-processed differential GPS. Subsequent analysis involved the calculation of residual values produced by comparing the computer-generated surface with a set of test points measured using differential GPS. This research demonstrates that the optimal digital system matched the performance of analytical photogrammetry for the collection of geomorphological data.