We use unmanned aerial vehicles (UAV) with high-accuracy control points to obtain orthophoto mosaics and digital surface models (DSM) by Pix4Dmapper in parts of the Mao-Luo stream and Hui-Sun forestry areas, Nantou County. Two different rotor UAVs were used to carry out the missions in the Mao-Luo stream area. The results were used to estimate the river channel sediment variation from April 2015 to April 2016, and were also evaluated with
ground check points with both horizontal and vertical components. A fixed-wing UAV was used for the mission in the Hui-Sun forestry area. The accuracies of the results using different distributions of control points were also computed and analyzed with both horizontal and vertical components. The main results of this study include: (1) The DSM derived from the 4 rotor UAV with a GoPro yields the accuracies of 9.2, 5.3, 4.8 cm at directions of X, Y, and Z; (2) The DSM derived from the 6 rotor UAV with a Sony NEX7 camera yields the accuracies of 3.0, 4.1, 1.6 cm at directions of X, Y, and Z; (3) The river channel sediment variation from April 2015 to April 2016 is 225,685.61 m3, and the internal accuracy for 1 m3 earthwork reaches 0.202 m3; (4) In Hui-Sun forestry area, the accuracy of UAV-derived DSM using control points distributed averagely is obviously better than that using control points distributed over low elevations only.
Key Words: UAV, DSM, accuracy analysis.