Accuracy Evaluation of Dems Derived from UAV-Data Acquired Over a Narrow-Deep Valley in the Middle-Atlas Mountain

The purposes of this paper is an evaluation of the accuracy of DEMs and associated products derived from UAV-data acquired at two different flight altitudes over a narrow-deep mountainous valley area. The assessment of quality and accuracy were undertaken, respectively, qualitatively by reference to the field observations and quantitatively using DGPS in situ measurements. The assigned time for processing and products derivations steps was also analyzed. The images were acquired using UAV Mavic Pro-2 RGB digital camera at two flight altitudes of 80 and 150 m providing, respectively, 2 dissimilar spatial resolutions, 2.0 and 3.5 cm. The "PIX4D-Mapper" software was used for photogrammetric processing. Uniformly distributed over the study area, 35 GCPs were measured with accurate 500-RTK GPS (σ ≤ ± 1 cm). Among them, 26 points were applied for the calibration procedure and 9 as checkpoints. The results demonstrated that the low altitude flight at 80 m enabled the acquisition of 482 overlapping images that were processed during 7 h 41 min, yielding an RMSE ˂ ± 2.0 on planimetry and ˂ ± 2.5 cm on altimetry of checkpoints. Whereas, the flight at 150 m allowed the recording of 250 overlapping images which were processed during 3 h 22 min, reaching an RMSE ˂ ± 3.0 and ˂ ± 3.7 cm on planimetry and altimetry, respectively. Visual analysis pointed out that regardless the considered altitude, the derived products (DEMs, DSMs and ortho-image) are very similar and faithfully reflect the reality on the ground. Accordingly, a flight at 150 m is able to provide satisfactory accuracies of DEM and auxiliary products generated from UAV Mavic Pro-2 camera data for applications in narrow-deep mountain valleys. However, an experienced operator of drone flight control is required to avoid the risks of accidents when maneuvering between narrow rock walls and to land successfully the drone in a safe place. Moreover, these operations are also conditioned by the reliability of telemetry control and the availability of GPS (or GNSS) signals in this harsh environment.