Abstract. The spatial distribution and size of avalanches are essential parameters for
avalanche warning, avalanche documentation, mitigation measure design and
hazard zonation. Despite its importance, this information is incomplete
today and only available for limited areas and limited time periods. Manual
avalanche mapping from satellite imagery has recently been applied to reduce
this gap achieving promising results. However, their reliability and
completeness have not yet been verified satisfactorily. In our study we attempt a full validation of the completeness of visually
detected and mapped avalanches from optical SPOT 6, Sentinel-2 and radar
Sentinel-1 imagery. We examine manually mapped avalanches from two avalanche
periods in 2018 and 2019 for an area of approximately 180 km2 around
Davos, Switzerland, relying on ground- and helicopter-based photographs as
ground truth. For the quality assessment, we investigate the probability of
detection (POD) and the positive predictive value (PPV). Additionally, we
relate our results to conditions which potentially influence avalanche
detection in the satellite imagery. We statistically confirm the high potential of SPOT for comprehensive
avalanche mapping for selected periods (POD = 0.74, PPV = 0.88) as well
as the reliability of Sentinel-1 (POD = 0.27, PPV = 0.87) for which the
POD is reduced because mainly larger avalanches are mapped. Furthermore, we
found that Sentinel-2 is unsuitable for the mapping of most avalanches due
to its spatial resolution (POD = 0.06, PPV = 0.81). Because we could
apply the same reference avalanche events for all three satellite mappings,
our validation results are robust and comparable. We demonstrate that
satellite-based avalanche mapping has the potential to fill the existing
avalanche documentation gap over large areas, making alpine regions safer.