An evaluation of a low-cost pole aerial photography (PAP) and structure from motion (SfM) approach for topographic surveying of small rivers

To better understand fluvial forms and processes, geomorphologists have a need for high-resolution fluvial topographic surveys. Continuous data collection approaches such as airborne laser scanning (ALS), terrestrial laser scanning (TLS) and structure from motion (SfM) photogrammetry provide methods to collect such data sets. Comparisons of SfM and laser-based approaches for topographic surveys have demonstrated site-specific benefits and drawbacks. Survey preference is largely dependent on specific project requirements, indicating a need for examples of best practice for different applications. This study demonstrates how pole aerial photography (PAP) provides specific advantages for SfM data collection when mapping the topography of small river corridors. Digital elevation models (DEMs) were created using three different surveying approaches for a 100 m reach of Coledale Beck, a small upland stream in Cumbria, United Kingdom. This included (a) imagery collected from a 5 m telescopic pole processed using SfM photogrammetry, (b) imagery collected using an unmanned aircraft system (UAS), also processed using SfM photogrammetry, and (c) point cloud data collected using a TLS. All three approaches produce DEMs of sufficient quality to enhance our understanding of fluvial forms and processes, with DEM/point cloud resolutions of 0.01 m for the close-range methods (TLS and PAP) and c. 0.02 m for the longer-range UAS method. Overall, TLS mean errors (0.123-0.135 m) are almost twice as large as the UAS and PAP (0.037-0.103 m) errors and the standard deviation is approximately 25% higher. However, results vary significantly for different surface cover types (i.e. vegetated, exposed and submerged surfaces), with TLS outperforming the other approaches for exposed gravel surfaces. Data acquisition rates for the PAP approach are approximately half those of the two other methods (430 m2/hour versus 845 and 730 m2/hour for PAP, TLS and UAS respectively). When equipment costs and ease of use are taken into consideration, the PAP approach provides an effective way of collecting topographic data from small rivers.