Techniques for Comparing Multi-Temporal Archive Aerial Imagery for Glacier Monitoring With Poor Ground Control

Abstract

Abstract. The measurement of geometric changes in Alpine glaciers is an essential aspect to assess their reaction to climate change effects. Archive aerial images may integrate valuable information to this purpose at times when other types of remotely sensed data are not available. The application of photogrammetric techniques such as Structure-from-Motion (SfM) and Multi-View-Stereo matching allows the extraction of dense point clouds to model the glacier environment. The comparison of multiple datasets requires to setup a stable reference system, a task that in archive photos is generally carried out by means of ground control points (GCPs). This paper would like to propose and assess some techniques to cope with the lack of ground control. Multitemporal SfM (MSfM) is presented and tested on a dataset including six different aerial blocks collected by means of analogue (PAN/RGB) and digital airborne cameras from 1967 to 2006. These images have been downloaded from the IGNF online repository and cover the area of the Val Veny (Brenva and Miage glaciers) in the Mount Blanc massif, at the border between Italy and France. Coupled with other solutions (i.e., extraction of GCPs from maps and DTMs and ICP co-registration of point clouds), MSfM has revealed as a suitable technique for coregistration of multiple photogrammetric blocks of aerial photos with minimum ground control. Some tests carried out in the case study area demonstrated that the integration of MSfM and ICP coregistration for refinement may significantly improve the comparison between multiple point clouds, which is a fundamental pre-requisite for the analysis of glacier changes over time.

Multitemporal Structure-from-Motion: A Flexible Tool to Cope with Aerial Blocks in Changing Mountain Environment

“…In this article the focus is given to the Multitemporal Structure-from-Motion (MSfM), which is presented here as a solution to obtain an accurate coregistration between photogrammetric blocks captured at different epochs. While this approach was already presented by the authors to deal with multitemporal data sets of aerial archive photos (Scaioni et al, 2023), here the methodology is reported with the general purpose to align any type of aerial data, including the integration of different data types in the same project (see Sect. 2).…”

“…The processing pipeline described in Subsection 2.2 for MSfM has been applied to the aerial blocks in Table 1. More details can be found in Scaioni et al (2023). A set of seven GCPs has been initially used for georeferencing all blocks in the reference frame RDN 2008.0 / UTM32N.…”

“…Results of comparisons between consecutive epochs are reported in Table 3, that shows also the outcomes based on the independent SfM per each epoch, and a third approach where MSfM is followed by a refinement of point cloud registration. This refinement has been applied based on the Iterative Closest Point algorithm (ICP -see Pomerleau et al, 2013) in SAs, as described in Scaioni et al (2023). As can be seen in Table 3, MSfM allows to reduce the absolute value of the mean of M3C2 distances between consecutive point clouds.…”