UAV RTK/PPK Method—An Optimal Solution for Mapping Inaccessible Forested Areas?

Abstract

Mapping hard-to-access and hazardous parts of forests by terrestrial surveying methods is a challenging task. Remote sensing techniques can provide an alternative solution to such cases. Unmanned aerial vehicles (UAVs) can provide on-demand data and higher flexibility in comparison to other remote sensing techniques. However, traditional georeferencing of imagery acquired by UAVs involves the use of ground control points (GCPs), thus negating the benefits of rapid and efficient mapping in remote areas. The aim of this study was to evaluate the accuracy of RTK/PPK (real-time kinematic, post-processed kinematic) solution used with a UAV to acquire camera positions through post-processed and corrected measurements by global navigation satellite systems (GNSS). To compare this solution with approaches involving GCPs, the accuracies of two GCP setup designs (4 GCPs and 9 GCPs) were evaluated. Additional factors, which can significantly influence accuracies were also introduced and evaluated: type of photogrammetric product (point cloud, orthoimages and DEM) vegetation leaf-off and leaf-on seasonal variation and flight patterns (evaluated individually and as a combination). The most accurate results for both horizontal (X and Y dimensions) and vertical (Z dimension) accuracies were acquired by the UAV RTK/PPK technology with RMSEs of 0.026 m, 0.035 m and 0.082 m, respectively. The PPK horizontal accuracy was significantly higher when compared to the 4GCP and 9GCP georeferencing approach (p < 0.05). The PPK vertical accuracy was significantly higher than 4 GCP approach accuracy, while PPK and 9 GCP approach vertical accuracies did not differ significantly (p = 0.96). Furthermore, the UAV RTK/PPK accuracy was not influenced by vegetation seasonal variation, whereas the GCP georeferencing approaches during the vegetation leaf-off season had lower accuracy. The use of the combined flight pattern resulted in higher horizontal accuracy; the influence on vertical accuracy was insignificant. Overall, the RTK/PPK technology in combination with UAVs is a feasible and appropriately accurate solution for various mapping tasks in forests.

Coastal Mapping Using DJI Phantom 4 RTK in Post-Processing Kinematic Mode

“…The results show that DJI-P4RTK surveying methodologies exclusively based on the acquisition of aerial images (following a specific protocol, such as the recording of GNSS data, an accurate image capture timestamp, and a few oblique images in addition to nadiral ones) are able to produce photogrammetric models with a centimeter-level accuracy, even if no GCPs are placed or surveyed or if no local base receiver is used. This aspect becomes particularly relevant whenever the time that is actually available for the survey is very limited or the site is not accessible due to safety reasons [31,32], coastal pollution [33], or remote forested areas [34].…”

Current Practices in UAS-based Environmental Monitoring

“…Currently, onboard differential global navigation satellite system (DGNSS) techniques are being adopted for UAS-photogrammetry, to provide high-quality camera position measurements and enable direct georeferencing to centimeter-level accuracy [72][73][74]. Such techniques require two GNSS receivers: a base station receiver or a service of continuously operating reference stations (CORS), to provide differential correction data, along with a rover GNSS receiver mounted on the UAS.…”

“…The RTK/PPK method is less influenced by onboard flight patterns, vegetation phenology, and data post-processing. Notably, the RTK/PPK method succeeds in surveying inaccessible or hazardous terrains, such as dense forests [72], volcano tectonics [78], or settlement landfills [79] and ice sheets [80,81]. A recent study by Zhang et al [82] showed that the PPK method can provide the same accuracy as the use of GCPs, with a small variation in vertical accuracy, which could be solved by including only one GCP.…”

Volcanological applications of unoccupied aircraft systems (UAS): Developments, strategies, and future challenges

“…Eling et al 2015;Turner et al 2014]), then this information can be included within the photogrammetric processing in order to help reduce systematic error, as well as to provide accurate georeferencing. Even if highaccuracy camera position data are available, the use of at least one GCP is key to constraining vertical accuracy [Benassi et al 2017;Forlani et al 2018;Gerke and Przybilla 2016;Grayson et al 2018;Tomaštík et al 2019]. Furthermore, for all scenarios, in order to reliably quantify the accuracies achieved, some GCPs should be excluded from the processing, and used as independent check points for validation [Chandler 1999].…”