Abstract
The application of structure-from-motion (SfM) to generate digital terrain models (DTMs) derived from different image sources has strongly increased, the major reason for this being that processing is substantially easier with SfM than with conventional photogrammetry. To test the functionality in a demanding environment, we applied SfM and conventional photogrammetry to archival aerial images from Zmuttgletscher, a mountain glacier in Switzerland, for nine dates between 1946 and 2005 using the most popular software packages, and compared the results regarding bundle adjustment and final DTM quality. The results suggest that by using SfM it is possible to produce DTMs of similar quality as with conventional photogrammetry. Higher point cloud density and less noise allow a higher ground resolution of the final DTM, and the time effort from the user is 3–6 times smaller, while the controls of the commercial software packages Agisoft PhotoScan (Version 1.2; Agisoft, St. Petersburg, Russia) and Pix4Dmapper (Version 3.0; Pix4D, Lausanne, Switzerland) are limited in comparison to ERDAS photogrammetry. SfM performs less reliably when few images with little overlap are processed. Even though SfM facilitates the largely automated production of high quality DTMs, the user is not exempt from a thorough quality check, at best with reference data where available. The resulting DTM time series revealed an average change in surface elevation at the glacier tongue of −67.0 ± 5.3 m. The spatial pattern of changes over time reflects the influence of flow dynamics and the melt of clean ice and that under debris cover. With continued technological advances, we expect to see an increasing use of SfM in glaciology for a variety of purposes, also in processing archival aerial imagery.
Highlights
Digital terrain models (DTMs), as representations of the earth’s surface, are an indispensable source of information in the geosciences
23 out of 27 possible digital terrain models (DTMs) could be generated with a satisfying level of quality, i.e., an RMS error that is lower than the glacier signal for each period and a ground resolution that enables the analysis of change patterns (Table 2)
Using the root mean square error (RMSE), we found no significant difference between different software
Summary
Digital terrain models (DTMs), as representations of the earth’s surface, are an indispensable source of information in the geosciences. Many studies have a methodological nature, focusing on the new technology and its possibilities, whilst others applied SfM-MVS, e.g., to geomorphological [3,4] and glaciological [5,6] problems. Small areas (102–104 m2) are considered and terrestrial or low-altitude image platforms such as UAVs, blimps and kites are used. Bakker et al (2016) [3] have investigated whether SfM-MVS can be applied to plane-based aerial images. This is more challenging due to fewer images, a minor change in viewing angle (usually nadir), and a smaller image overlap. The accuracies of the relatively small area of interest (AOI), a braided riverbed, were comparable, and changes in the order of decimetres could be detected
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