Abstract. Monitoring agricultural areas threatened by soil erosion often requires
decimetre topographic information over areas of several square kilometres.
Airborne lidar and remotely piloted aircraft system (RPAS) imagery have the
ability to provide repeated decimetre-resolution and -accuracy digital
elevation models (DEMs) covering these extents, which is unrealistic with
ground surveys. However, various factors hamper the dissemination of these
technologies in a wide range of situations, including local regulations for
RPAS and the cost for airborne laser systems and medium-format RPAS imagery.
The goal of this study is to investigate the ability of low-tech kite aerial
photography to obtain DEMs with decimetre resolution and accuracy that permit
3-D descriptions of active gullying in cultivated areas of several square
kilometres. To this end, we developed and assessed a two-step workflow.
First, we used both heuristic experimental approaches in field and
numerical simulations to determine the conditions that make a photogrammetric flight possible and
effective over several square kilometres with a kite
and a consumer-grade camera. Second, we mapped and characterised the entire
gully system of a test catchment in 3-D. We showed numerically and
experimentally that using a thin and light line for the kite is key for
a complete 3-D coverage over several square kilometres.
We thus obtained a decimetre-resolution DEM covering 3.18 km2 with a
mean error and standard deviation of the error of +7 and
22 cm respectively, hence achieving decimetre accuracy. With this data set,
we showed that high-resolution topographic data permit both the detection and
characterisation of an entire gully system with a high level of detail and an
overall accuracy of 74 % compared to an independent field survey. Kite
aerial photography with simple but appropriate equipment is hence an
alternative tool that has been proven to be valuable for surveying gullies
with sub-metric details in a square-kilometre-scale catchment. This case
study suggests that access to high-resolution topographic data on these
scales can be given to the community, which may help facilitate a better
understanding of gullying processes within a broader spectrum of conditions.
Abstract. 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 5 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 10m . Geographical reference was given by 8 ground points and 469 independent points were surveyed for validation. Estimated mean error on altitudes was 0.07m and standard deviation of this error 0.22m, for a 0.11m 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 15 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 20 square watershed with kite-borne imagery.
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