3.2.3. Ground Control Measurements

Reference points were deployed and measured in all three case locations to include known point coordinates for georeferencing as well as a means for quality control. Clearly visible ground marker had a quadratic shape with an edge length of 30 cm showing a black and white chess pattern (cf. Figure 4) which were evenly distributed in the area of interest. As specified in Table 4, ground truth measurements were carried out with two different GNSS devices. The first was a pair of Leica CS10 stations used in a base and rover set with a final RTK measurement accuracy of 2 cm. The second device used was a handheld Trimble GeoXH receiving RTK corrections via the Rwandan Continuously Operating Reference Station (CORS) GeoNet with a final measurement accuracy of 10 cm. Whereas GCPs were included as a weighted observation during the photogrammetric image processing [42], checkpoints were not taken into account during image processing and present as a classical way to evaluate the geometric accuracy. The georeferenced orthomosaic has been generated following two different block orientation methods. The Gahanga dataset was processed by means of an integrated sensor orientation method [43] that uses the information of camera positions and attitude as well as object coordinates of GCPs for the Bundle Block Adjustment. Since no attitude measurements were available for the FireFLY6 and for the Inspire 2, the block orientation of the Muhoza and Busogo dataset followed the GNSS-supported Aerial Triangulation method based on information on camera positions and object coordinates of GCPs [cf. 43].

**Figure 4.** Measurement of reference points.


Although less than ten reference points are sufficient to achieve high geometric accuracies, redundancy in deployed points has proven to be the preferable option as ground marker might get vanished or destroyed. Due to unforeseen administrative problems, the time between the deployment of the ground marker and the UAS flight itself was almost 5h. This can explain the fact that nearly 25% of all deployed points in the area of Muhoza were taken away. As summarised in Table 5, in the peri-urban and rural areas of Busogo and Gahanga the authors experienced less time delay as well as fewer losses of ground marker.

**Table 5.** Number of deployed reference points - count before and after the UAS flight.

