Abstract
Digital terrain model (DTM) generation is essential to recreating terrain morphology once the external elements are removed. Traditional survey methods are still used to collect accurate geographic data on the land surface. Given the emergence of unmanned aerial vehicles (UAVs) equipped with low-cost digital cameras and better photogrammetric methods for digital mapping, efficient approaches are necessary to allow rapid land surveys with high accuracy. This paper provides a review, complemented with the authors’ experience, regarding the UAV photogrammetric process and field survey parameters for DTM generation using popular commercial photogrammetric software to process images obtained with fixed-wing or multicopter UAVs. We analyzed the quality and accuracy of the DTMs based on four categories: (i) the UAV system (UAV platforms and camera); (ii) flight planning and image acquisition (flight altitude, image overlap, UAV speed, orientation of the flight line, camera configuration, and georeferencing); (iii) photogrammetric DTM generation (software, image alignment, dense point cloud generation, and ground filtering); (iv) geomorphology and land use/cover. For flat terrain, UAV photogrammetry provided a horizontal root mean square error (RMSE) between 1 to 3 × the ground sample distance (GSD) and a vertical RMSE between 1 to 4.5 × GSD, and, for complex topography, a horizontal RMSE between 1 to 7 × GSD and a vertical RMSE between 1.5 to 5 × GSD. Finally, we stress that UAV photogrammetry can provide DTMs with high accuracy when the photogrammetric process variables are optimized.
Highlights
Many applications require the generation of digital terrain models (DTMs), generated by the interpolation of points belonging to the bare land surface [1] from altimetric data produced from conventional or advance survey methods
These, and many other studies, show that unmanned aerial vehicles (UAVs)-based aerial photogrammetry can be competitive in terms of the accuracy, spatial resolution, automation, and costs compared with other techniques, such as Laser Imaging Detection and Ranging (LiDAR) [25], terrestrial laser scanners (TLS) [2], total stations (TS) [18], or Global Navigation Satellite Systems (GNSS) [26], for certain applications, as long as specific procedures and survey parameter optimization are followed
The goal of this study is to present recent advances, complemented with the authors’ experience, regarding aerial photogrammetric procedures and the quality of field survey parameters that must be taken into account to improve DTM generation with low-cost UAV platforms
Summary
Many applications require the generation of digital terrain models (DTMs), generated by the interpolation of points belonging to the bare land surface [1] from altimetric data produced from conventional or advance survey methods. These, and many other studies, show that UAV-based aerial photogrammetry can be competitive in terms of the accuracy, spatial resolution, automation, and costs compared with other techniques, such as LiDAR [25], TLS [2], TS [18], or GNSS [26], for certain applications, as long as specific procedures and survey parameter optimization are followed. Governmental agencies can establish limits for their product specifications and applications and contracting purposes In this sense, the accuracy of the DTM is one of the more pressing concerns, and it highly depends on the quality of the field survey. DTM generation appears to be a simple process; high DTM accuracy demands the optimization of photogrammetric procedures and survey parameters and follows essential UAV flight rules [3,5]. In the Conclusion, our primary recommendations to obtain high quality DTM, as derived from the work, are presented
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