# Quality Assessment of DSMs Produced from UAV Flights Georeferenced with On-Board RTK Positioning
### GNSS-Assisted Block Georeferencing and Control  
The measurement of GCP coordinates is often today the most time-consuming on-site operation of an UAV photogrammetric survey. Artificial targets (even coded ones that are automatically recognised by the processing software) are necessary, as finding well-defined natural features of appropriate size to be used as GCP in pre-flight or post-flight surveys in outside urban areas might be difficult. GCPs are normally measured with GNSS surveys in real-time kinematic (RTK) mode, either with respect to a nearby base station or with differential corrections being sent by a Continuously Operating Reference Stations (CORS) Network (NRTK). Both techniques are accurate to within 1-2 cm in horizontal coordinates and 2-3 cm in elevation (i.e., to a level that often matches the GSD). Dispensing with GCP altogether would be a big step forward for UAV photogrammetric surveys, not only for the time savings in the field and the office. The flexibility of the platform is not always matched by the ground operations. Sometimes, targets cannot be placed or measured easily (e.g., in areas with trees or dense bushes, or on mountain slopes) or safely (on a landslide's body or on a glaciated area), or may interfere with ongoing activities (as in large open pit mines). However, ground control is not the only option for block georeferencing and block deformation control. An alternative is available, if the drone on-board GNSS receiver is capable of delivering cm-level positioning accuracy. GNSS-assisted block orientation (AAT, Assisted Aerial Triangulation for short in the following) is a technique that has been used in aerial photogrammetry for many years [33; 34]. In AAT, the coordinates of the camera stations at the exposure time are measured by an on-board GNSS receiver, and introduced as constraints in the BBA, or used to compute the transformation from the SfM arbitrary reference to the mapping system. As RTK-enabled, dual-frequency receivers are being made available on an increasing number of commercial UAV platforms, it is important to investigate the error propagation of this alternative georeferencing technique. Recent simulations [35; 36] suggest that block deformations are effectively kept in check by AAT. Previous empirical evidence [37; 38; 39; 40] suggest that the traditional control through GCP is still the safest and most accurate way to georeference a UAV photogrammetric block. As far as DSM generation by AAT is concerned, an elevation root mean square error (RMSE) of about 9 cm was reported on 180 checkpoints [40], versus 3 cm by georeferencing with GCP. In order to exploit the accuracy potential of AAT, system calibration is required. The receiver and the camera acquire data asynchronously, while the GNSS position refers to the antenna phase center, and rather than the camera center. Therefore, the exposure time of each image must be recorded to interpolate the trajectory, and the offset between the camera projection center and the antenna phase center [37] must be taken into account. Residual errors in camera interior orientation parameters are likely to affect the ground coordinates [41] rather than being partly absorbed by exterior orientation parameters, as happens in GCP-controlled radial blocks. An accurate camera calibration is therefore essential if no GCP are available. This problem is also shared by direct georeferencing [42; 43], a technique coupling a GNSS receiver and an inertial measurement unit (IMU) to determine both camera position and attitude at shooting time. However, as far as DSM generation is concerned, direct georeferencing is unlikely to remove SfM from the UAV photogrammetry workflow, as tie points are needed in dense matching anyway. Though cm-level accuracy can also be provided by single-frequency receivers, dual-frequency ones increase system reliability, as they solve the integer ambiguity faster [44]. RTK positioning allows image geotagging and on-site data quality checks.