Potential of kite-borne photogrammetry for decimetric and kilometre square 3D mapping: an application for automatic gully detection

This work proposes an alternative method to answer the issue of quasi-exhaustive mapping of erosion features on kilometre square areas by remote sensing. This study presents a method to produce decimetric Digital Elevation Models (DEMs) with kite aerial photography and an algorithm to map gully erosion from these DEMs. Kite aerial photography is robust and cheap in comparison to Unmanned Aerial Vehicles (UAVs). The use of such a simple apparatus is made possible if the flight angle of the kite is steady. Experimentation and modelling show that this goal can be reached with these two predetermined conditions: the right kite must be used in the right wind and the line must be light and thin. In our study, we used a 10 m 2 framed delta kite in 4–5 Beaufort winds using a Dyneema ® line of 1 mm in diameter and 0.5 g/m in weigth for image acquisition on the day of experiment. Within two successive flights, 752 images were acquired. The photogrammetric software used was Micmac, an open-source software written and maintained by the French national institute of geographic and forest information (IGN). It allowed to obtain DEM covering 3.18 km 2 . Geographical reference was given by 8 ground points and 469 independent points were surveyed for validation. Estimated mean error on altitudes was 0.07 m and standard deviation of this error 0.22 m, for a 0.11 m ground sampling distance. In order to illustrate the potential of such detailed DEMs at the watershed scale, a simple gully detection algorithm was briefly described and implemented. As with several others, the method does not refer to the relationship between slope and drainage area but uses local convolution of the DEM. Considering a smoothed DEM as a proxy of the geomorphological process of gullies healing, proposed gully detection algorithm relies on substracting smoothed DEM from the original DEM. The depth of each feature is then estimated and only the bulkier elements are kept as potential gullies. Despite our algorithm detecting undesired artefacts – most of them being man-made structures such as houses and roads – all gully heads and all channels are detected. Therefore results show the benefits of the production and use of decimetric DEMs on an entire kilometre square watershed with kite-borne imagery.