Alpine glacier surface velocity measurement from UAV imagery – examining the effect of image resolution on the accuracy of results

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The reliability and validity of the Glacier Surface Velocity (GSV) measurement results based on remote sensing datasets depends on the quality and spatial resolution of the image used. The typical pixel size of space-borne satellite imagery is often larger than the annual and inter-annual displacement of small alpine debris-covered glaciers. In addition, the pixel size of medium resolution satellite images (10–30 m), limits the size of a feature that can be matched. This is even more of an issue for glaciers located in arid and semi-arid environments (e.g. glaciers in Iran and high mountains of Asia) where flow velocities are not exceeding a few metres per year. Consequently, high-resolution data such as Unmanned Aerial Vehicle (UAV) images are required to calculate the surface velocity of such glaciers. However, the optimal resolution of UAV images is one of the most important challenges in GSV measurement. This paper explores the influence of UAV image resolution on the quality of GSV results. Analysis was carried out at 19 different resolutions of UAV images from 10 to 100 cm with a 5 cm bin over a debris-covered glacier, in Iran. COSI-Corr algorithm was used to perform the image correlation. To evaluate the accuracy of obtained results, manual digitisation was performed and the differences between manual GSV and frequency cross-correlator were assessed for all data sets. Moreover, displacement calculated for a stagnant off-glacier area was evaluated. While the quality of the results of the images between 10 and 30 cm is substantially the same, the obtained results indicate that the best result of GSV was not obtained using the finest image resolution. Results revealed that the highest correspondence between the measured GSV and manual digitisation was obtained in a 30 cm spatial resolution image. In addition, the 30 cm image resolution shows the minimum uncertainty over the off-glacier static area. Obtained results revealed that using too fine resolution images will lead to computational redundancy, while no improvement is observed in the accuracy of GSV results.