Retrieval algorithm

Glacier area outline (GAO) is mapped using manual digitation from topographic maps and segmentation of ratio images for satellite imagery (Landsat TM/ETM+/OLI, Seninel-2). Manual edits are needed to correct for seasonal snow, debris, lakes and parts in shadow.

Glacier lake outline (GLO) is mapped using a combination of methods: normalized difference water index (NDWI), manual digitisation, band thresholding and by map overlay with lakes from topographic digital maps (Andreassen et al. 2021).

The algorithms required to effectively extract glacial variables from satellite images depend upon illumination conditions, landscape types and scene conditions. Manual adaption is needed to check the suitability of algorithms for each region and scene.

Validation

The applicability of standard glacier mapping methods is tested using orthophotos from norgeibilder.no and other sources. Results so far confirm that the glacier outlines and lake outlines can be mapped accurately, but that the automatic mapping often needs manual corrections. Automatically derived outlines are therefore checked against the satellite imagery, orthophotos and topographic maps, and manual corrections applied when needed.

Reference:

Andreassen, L. M. (ed.), G. Moholdt, A. Kääb, A. Messerli, T. Nagy and S.H. Winsvold, 2021. Monitoring glaciers in mainland Norway and Svalbard using Sentinel. NVE Rapport 3-2021, 94 s. https://publikasjoner.nve.no/rapport/2021/rapport2021_03.pdf

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