Evaluation of different GNSS solutions and SFM software workflows for surveying shorelines and remote areas using UAS
The emergence and modernization of Unoccupied Aircraft Systems (UAS), broadly known as drones, and Structure-from-Motion (SfM) photogrammetry have made significant contributions to the geospatial and surveying world. Traditionally, indirect georeferencing by using ground control points (GCPs) is used to georeference UAS imagery when high accuracy positioning is required. However, this approach is tedious and impractical when surveying remote or inaccessible coastal areas, or when desiring to map coastlines from shipborne UAS operations. The broad applicability of UAS and SfM technologies has led to a wide range of data collection and SfM processing workflows that can be utilized, enhanced further by the implementation of various Global Navigation Satellite Systems (GNSS) techniques for direct georeferencing of the imagery. As part of an investigation conducted by the Office of Coast Survey (OCS) at the National Oceanic and Atmospheric Administration (NOAA), this study seeks to identify UAS-SfM data collection and processing workflows that maintain vertical accuracies at the decimeter-level without the aiding of GCPs. The study uses UAS imagery collected from two different UAS platforms at two different sandy beach study sites along the Southern Texas Gulf Coast. The objectives of the study are two-fold: (i) examine the applicability of Real-Time Kinematic (RTK), Post-Processed Kinematic (PPK), and Precise Point Positioning (PPP) GNSS solutions as plausible substitutes to ground control points (GCPs) for UAS-SfM shoreline mapping, and (ii) to evaluate the impact of three-commercial SfM software (Drone2Map, Metashape, and Pix4D) and one open-source software (Web OpenDroneMap) on the quantitative and qualitative characteristics of resulting mapping products. Results showed that RTK and PPK can reach centimeter-level vertical accuracies, fulfill the requirements set forth for this project, and are the most suitable alternatives to GCPs for remote surveying when plausible. When using PPK, the highest accuracies were reached when using base stations within 30 kilometers of the survey site, especially when combined with higher percentages of PPK fix, a measure that explains the number of photos that successfully underwent PPK correction. PPP offers the best alternative for remote UAS surveying, given that it is a single - receiver method, but the results evaluated here did not meet desired vertical accuracy levels. However, enhancing convergence time techniques is likely to reach even better results. In terms of SfM software, Metashape and Pix4D proved to be the most robust software altern atives achieving repeatable centimeter-level vertical accuracies for derived mapping products. Several inconsistencies were observed with Drone2Map and ODM, which hinder its applicability for UAS surveying without GCPs. The results and techniques discussed in this study help to optimize data acquisition and processing workflows for shoreline mapping and remote surveying.