APPLICATION OF 'STRUCTURE FROM MOTION' PHOTOGRAMMETRY TO RIVER CHANNEL CHANGE STUDIES

This study considers the feasibility and accuracy of using the Structure from Motion (SfM) technique to quantify changes in stream channel morphology. The SfM method utilizes common points across multiple photographs to create a 3-D representation of a study area. This model can then be georeferenced using ground control points. The camera locations and optics do not need to be known in this technique, making it simpler to implement in the field than traditional photogrammetry or ground-based lidar methods. We apply the SfM method to a field site on the Souhegan River in southern New Hampshire, where we photographed two cross-sections (one boulder-bedded, one sand-bedded) using a camera mounted on a 4.7-m pole. On the same day, we surveyed both cross sections using a total station with mm-scale accuracy. Our initial results yield several findings. First, for the primarily boulder-bedded cross-section, the SfM software had difficulties establishing overlapping points in photographs. One possible solution for this problem would be to collect photographs from a higher altitude where more points above the water surface could be seen in each image. Second, at the sand-bedded cross-section, an exposed bar was rendered well by the SfM software. When compared to total station survey data, the SfM cross-section averaged 3.6 cm (±3.4 cm standard deviation) higher than the survey, based on a point-by-point comparison. Importantly, the SfM results provide a much larger data set, constructing an essentially continuous cross-section. Overall, initial results suggest SfM will be a powerful tool for future geomorphic studies, particularly in subaerial sites with relatively sparse vegetation.