Eratosthenes

Around 240 BC, Eratosthenes was the first scientist to estimate the size of the Earth by measuring the distance of shadows cast in a well. This project builds on his legacy, using repeat satellite imagery to detect shadows cast by mountain tops on glaciers. These shadows, when tracked between sun-synchronous satellite imagery, have the potential of being converted into multi-temporal elevation differences on glaciers with global coverage.

Glaciers and ice caps are sensitive indicators of climate change. Although they store a freshwater volume equivalent that is two orders of magnitude smaller than that of ice sheets, they contribute ~1/3 to present sea-level rise. Yet, their contribution in the recent past has the largest uncertainty among sea-level rise components. In-situ observations of glacier mass balance are limited, and adequate remote sensing techniques to obtain worldwide, repeat observations of small and medium-sized glaciers are currently unavailable.

In this project, we have detected shadows cast by mountain tops on glaciers in optical satellite imagery. We have developed tools to delineate and extract mountain shadow profiles from multi-spectral images (Sentinel-2), to refine shadow edges via active contours and to match shadow profiles in time series. The resulting tool is thus a versatile geodetic imaging toolbox.