Comparison of Measurement Accuracy for DSM and Orthomosaic between Different UAV Flight Observations

This study compares the measurement accuracy of the digital surface model (DSM) and orthomosaics measured by an unmanned aerial vehicle in 2013 and 2016. The flight heights and resolutions of the aerial photographs were 60–137 m and 3 cm, respectively, in 2013, and 43 m and 1 cm, respectively, in 2016. The number of aerial photographs covering the study area was 109 in 2013 and 498 in 2016. The DSM and orthomosaics were produced from a dense point cloud pro-cessed by Structure from Motion on the software PhotoScan. Both productions were geo-referenced to the coordinates of four ground control points (GCPs) provided by the real-time kinematic-global navigation satellite system (RTK-GNSS) survey, which collects the three-dimensional coordinates (easting, northing, and elevation) on the four GCPs and the eval-uation points (EPs). After measuring the horizontal coordinates and elevations of the EPs on the orthomosaic and DSM, the measurement result was subtracted from the GNSS-surveyed result. The horizontal and vertical root mean square errors were 0.45 and 0.32 m, respectively, in 2013, and 0.05 and 0.02 m in 2016, respectively. Therefore, the horizontal and vertical accuracies of the observations were 9 and 16 times higher, respectively, in 2016 than in 2013.