Retreat of an Ice Wall in Dobšiná Ice Cave (2018–2025): A Comparison of Three Change-Detection Methods Based on SfM Photogrammetry

Perennial cave ice in the temperate zone is declining, and its monitoring requires suitable change-evaluation methods. This study compares three approaches to assessing the changes of a vertical ice wall in the Ruffínyi Corridor of Dobšiná Ice Cave (Slovakia), a UNESCO World Heritage Site, using a single multi-temporal dataset. Because the wall was inaccessible to tacheometry and laser scanning, we surveyed it using close-range Structure-from-Motion photogrammetry across seven annual epochs (2018–2023 and 2025). We evaluated changes relative to 2018 using cross-sections, M3C2 point cloud comparison, and vectorisation of five visible glacial layers. All three methods confirmed a predominant retreat of the wall. Cross-sections showed retreat of 40–176 mm (mean 113 mm) by 2025, the mean M3C2 distance reached −77.7 mm (≈12.9 mm/year), and the vectorised layers shifted by 72–214 mm (10–31 mm/year). Cross-sections and M3C2 independently identified a zone of local ice accumulation caused by water dripping from the ceiling (184 mm at one cross-section). The methods agree in the order of magnitude; differences reflect their spatial sampling. M3C2 provides the most complete area-wide assessment, cross-sections offer the simplest inspection and vectorisation, and a stratigraphically interpretable record; combined, they offer a more reliable basis for long-term monitoring of cave ice than any single method.