Multi-Scale Analysis of DEMs Derived from Unmanned Aerial Vehicle (UAV) in Precision Agriculture Context

In precision agriculture, accurate DEMs are very important for the characterisation of topographic attributes (elevation, slope, orientation, curvature, flow direction and accumulation) to understand the spatial variability at the field or within-field scale for managing several agricultural parameters including irrigation, fertilization, runoff production, etc. Unlike conventional methods, the synergy between UAV and digital photogrammetry is a new geomatics approach to generate DEMs with centimetre pixel size. This paper aims a multi-scale analysis of DEMs and related products (DSMs and orthomosaics) derived from UAV data acquired at three different flight altitudes over agricultural field. The quality and accuracies assessment 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-Camera (DJI Phantom 4 Pro+ V2.0) at three different flight altitudes (50, 100 and 150 m), providing three dissimilar spatial resolutions (1.25, 2.75 and 4.15 cm). The “Pix4Dmapper” software was used for photogrammetric processing. Uniformly distributed over the study area, 19 GCPs were measured with accurate DGPS (σ ≤ ± 1 cm), LEICA 500-RTK. Among them, 6 points were used for the calibration procedure and 13 as check-points. The results demonstrated that the low altitude flight at 50 m enabled the acquisition of 736 overlapping images that were processed during 5 h 55 min, yielding an RMSE < ± 2 on planimetry and < ± 3 cm on altimetry of check-points. Similar accuracies were achieved for the flight at 100 m, after processing 231 stereoscopic images during 2 h 26 min. Whereas, the flight at 150 m allowed the recording of 103 overlapping images which were processed during 1 h 46 min, reaching an RMSE of ± 2 and ± 4 cm on planimetry and altimetry, respectively. Visual analysis pointed out that regardless the considered altitude, the derived products (DEMs, DSMs and orthomosaics) are very similar and faithfully reflect the reality on the ground. Accordingly, a flight at 150 m is certainly able to provide satisfactory accuracies of DEM and auxiliary products generated from UAV-Camera (DJI Phantom 4 Pro+) data for applications in the context of precision agriculture.