Fisheye Photogrammetry to Generate Low-Cost DTMs

In the archaeological practice, Digital Terrain Models (DTMs) and Digital Surface Models (DSMs) may be used to represent spatial information about the site by conveying information such as differences in levels, morphology of the terrain and movements of volumes during the excavation. Nowadays DTMs and DSMs can be easily obtained by image-based matching using low altitude aerial dataset acquired from a digital camera by means of a lifting device. In recent years, the spread of commercial multi-rotor unmanned aerial vehicles and their decreasing cost made low-altitude aerial photography even easier than before, where balloons, kites and telescopic masts would have been used instead. However, the use of drones is often forbidden by law, especially in the archaeological areas, and therefore a more traditional approach must to be adopted instead. This paper presents two different approaches adopted on the field to acquire the DTM of an archaeological excavation: the use of a pole held by a chest harness to lift a camera up to 3,5m height fitted with a 20mm wide angle lens; and a second solution that exploits ground-based fisheye photogrammetry. In general, an image network acquired from ground level is challenging due to: i) the poor coverage that can be obtained on the ground, ii) the large number of images that are required to cover large areas and consequently iii) the longer elaboration time that is required to process the data. The fisheye approach, however, proved to be more effective thanks to the more robust image network resulting both from the wider field of view and from the possibility to handle large datasets by downsampling the images and still retrieving strong key points. The main difference with the first system is that the monotonous images acquired by the 20mm lens, very plain in texture, require working at full resolution in order to distinguish valid features in the sand. The final product of the tests carried out along this line in 2019 at Saqqara (Egypt) is a comprehensive DSM of the entire archaeological site with an accuracy of ~3cm.