The ground control points (GCPs) are used in the process of indirect georeferencing the Unmanned Aerial Systems (UAS) images. A minimum of 3 ground control points is required, but increasing the number of GCPs will lead to higher accuracy of the final results i.e. point cloud, 3D mesh, orthomosaic or DSM, if the GCPs are introduced as constraints in the bundle block adjustment process. Moreover, exceeding the number of ground control points is a time-consuming process, both in the field and computationally. The aim of the study is to provide the answer to the question of how many ground control points are necessary in order to derive high precision results. To obtain the results, an area of about 1.5 ha has been photographed with a low-cost UAS, namely DJI Phantom 3 Standard at two different heights: 28 m and 35 m above ground and a number of 50 ground control points, uniformly distributed over the study area, were measured using a total station. The flight planning was made using the Pix4D software, choo sing the longitudinal and transversal overlap of 80% and 40% respective ly, the camera being oriented in nadiral pos ition. The first flight was made in double grid, 122 images with the GSD of 1.2 cm being acquired and after the second flight, 51 images were acquired in a single grid, with the GSD of 1 cm. First, the UAS images were process using the Pix4D Mapper Pro software, using a minimum number of ground control points while the 47 remaining control points served as check points (CP) for accuracy assessment. So, the CPs were manually measured on each image, the coordinates being compared with the ones determined with high precision. Then, the number of GCP was gradually increased up to 40, the accuracy being checked on the remaining 10 CPs. The accuracy assessment, both in horizontal and vertical direction, with dense and well distributed GCP showed a RMS of 23.3 cm for a minimum of 3 GCPs while compared with the other cases, in which the RMS decreases under 6 cm after 10 GCPs. The second test was made with 3DF Zephyr software using a free-network approach in the bundle adjustment and only at the end of the bundle adjustment the GCPs are used to perform a similarity (Helmert) transformation in order to bring the image network results into the desired reference coordinate system. As already demonstrated in literature, if no constraint is introduced in the process of bundle adjustment, increasing the GCPs number will not improve the 3D shape of the surveyed scene, therefore, the accuracy of the georeferencing process by using different number of GCP must be evaluated in this situation too. Also, the point clouds and the mesh surfaces derived automatically after using the minimum and the optimum number of GCPs respectivel y, were compared with a TLS point cloud. The results expressed a clear overview on the number of GCPs needed for the indirect georeferencing process with minimum influence on the final results.
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