Boosting the Timeliness of UAV Large Scale Mapping. Direct Georeferencing Approaches: Operational Strategies and Best Practices

Abstract

The use of unmanned aerial vehicles (UAVs) is nowadays a standard approach in several application fields. Researches connected with these systems cover several topics and the evolution of these platforms and their applications are rapidly growing. Despite the high level of automatization reached nowadays, there is still a phase of the overall UAVs’ photogrammetric pipeline that requires a high effort in terms of time and resources (i.e., the georeferencing phase). However, thanks to the availability of survey-grade GNSS (Global Navigation Satellite System) receivers embedded in the aerial platforms, it is possible to also enhance this phase of the processing by adopting direct georeferencing approaches (i.e., without using any ground control point and exploiting real time kinematic (RTK) positioning). This work investigates the possibilities offered by a multirotor commercial system equipped with a RTK-enabled GNSS receiver, focusing on the accuracy of the georeferencing phase. Several tests were performed in an ad-hoc case study exploiting different georeferencing solutions and assessing the 3D positional accuracies, thanks to a network of control points. The best approaches to be adopted in the field according to accuracy requirements of the final map products were identified and operational guidelines proposed accordingly.

Uav Photogrammetry and VHR Satellite Imagery for Emergency Mapping. The October 2020 Flood in Limone Piemonte (Italy)

“…This issue is well described in other works (e.g. Forlani et al, 2018;Gabrlik et al, 2018;Teppati Losè et al, 2020b): the absence, or a low number, of GCPs can results in a wrong estimation of camera I.O. parameters, leading to lower accuracies of the overall photogrammetric processing.…”

“…For each of the seven areas, three different flights were carried out ( Table 1) for a total number of 21 flights that generated 5225 images. and a total number of 5225 images were acquired (red dots) (Peppa et al, 2019;Štroner et al, 2021;Taddia et al, 2020;Teppati Losè et al, 2020a, 2020b an NRTK-based DG solution enables centimeter-level accuracy in camera positioning and thus in the overall photogrammetric processing. Nevertheless, at least two different conditions should be met to adopt this approach: the presence of a network of CORSs (Continuously Operating Reference Stations) covering the area (including a subscription if required) and the presence of a stable Internet connection.…”

“…The DJI Phantom 4 RTK is actually provided with a camera calibration certificate, whose parameters are directly stored in the EXIF file (each image includes focal length, principal point, k1, k2, k3, p1, and p2). However, as reported in previous research work (Teppati Losè et al, 2020a, 2020b, the data provided by the company are not accurate enough to be used in the photogrammetric processing of a dataset using a precalibration approach for the solution of I.O. A self-calibration approach was thus adopted for the other configurations to evaluate how they differ from the precalibration certificate and between them.…”

Conventional and UAV-Based Aerial Surveys for Long-Term Monitoring (1954–2020) of a Highly Active Rock Glacier in Austria

“…While indirect georeferencing of UAV imagery is the method of choice in terms of the resulting quality (assessed at both independent check points and digital surface models (DSMs), e.g., Forlani et al, 2018), direct georeferencing (RTK/PPK) can result in qualities at the order of magnitude of only a few centimeters (Padró et al, 2019;Losè et al, 2020;Stott et al, 2020). However, large discrepancies can result in the viewing direction (i.e., vertical errors in nadir UAV-based surveys) for reasons including weak imaging configurations and non-metric cameras (e.g., Luhmann et al, 2014;Losè et al, 2020;Dall'Asta et al, 2017;Przybilla et al, 2020). In both direct georeferencing approaches, this effect can be easily mitigated using only a single GCP (Forlani et al, 2018;Zhang et al, 2019;Przybilla et al, 2020).…”

“…Rock glaciers are periglacial landforms in high mountain environments indicating permafrost conditions during their initial formation and entire evolution. In this context, permafrost is defined as ground material (soil or rock unrelated to the amount of ice and organic material) that remains at or below 0°C for at least two consecutive years (van Everdingen, 1998). Active rock glaciers contain permafrost and creep slowly down-valley by force of gravity, commonly forming flow structures with ridges and furrows like lava flows as a result of compressional forces (Barsch, 1996).…”