Application of digital photogrammetry to complex topography for geomorphological research

This paper is concerned with the application of automated digital photogrammetry, using 1:3000 scale photography to complex, natural landform surfaces, of typical interest to geomorphologists. It assesses the quality of the results obtained using a relatively cheap and readily available area based stereomatching package, in terms of precision, accuracy and external reliability. Precision is investigated with reference to the confidence that can be placed in individual matches. Accuracy is evaluated using specially collected, independent datasets obtained from an area of complex topography in Glen Affric, Scotland. Data collection was stratified to areas of different surface roughness. External reliability is judged with respect to estimates of slope, a key parameter in geomorphological investigations. The results show that, whilst the effects of grid density and vegetation correction are the most important controls upon the accuracy and the external reliability of the photogrammetric results, collection parameters associated with the stereomatching process can also exert some control, particularly in areas of complex topography. It is impossible to generalize rules for choice of optimal collection parameters without careful consideration of the surface under investigation. Given that maximum grid densities are defined by the object space pixel resolution, the paper concludes that surface quality is largely governed by traditional controls upon photogrammetric data quality (camera calibration, base:distance ratio, ground control), combined with either scanning density or digital image resolution. However, over some surfaces, careful consideration has to be given to the effect of matching parameters.