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Automatic classification of avalanche terrain


Terrain features, especially the slope angle and topography, are crucial factors in the assessment of avalanche danger and risk. Using a topographic map to understand the terrain is not easy, even for experts, and calls for a large measure of interpretation. In this context, high-resolution digital elevation models provide additional information about the terrain. Slope angle maps are already well established, but the SLF has now developed a method of producing new maps that classify avalanche terrain and give consideration to both the avalanche release and the avalanche runout zone. These maps give winter sport participants much more information than slope angle maps.


Would it be better to ascend by the steep ridge with a 40° slope angle, or by the less steep 35° slope? What distance should we maintain from the steep slope when traversing its base, in order to avoid remote triggering to every extent possible? Where is a good place to gather together for a rest with the least likelihood of being caught up in an avalanche? Is there a danger of being buried particularly deep, or of falling?

Answers to these questions depend on an assessment of both the avalanche and weather situation and the various aspects of the terrain. Unlike the snowpack, the terrain characteristics are subject to very little change. On the other hand, it is difficult even for experienced ski tourers to deduce the avalanche risk from an interpretation of the terrain. Digital elevation models with a high spatial resolution provide the basis for answering the above questions with computer support – for the entire territory of the Swiss Alps. The aim of the SLF project was to use these models to classify the avalanche territory, fully automatically and over a large area, as regards typical skier-triggered avalanches.

The analysis focused on the following aspects:

  • Terrain largely immune to avalanches
  • Typical release zones for slab avalanches
  • Terrain in which slab avalanches can be triggered remotely from the base of a slope
  • Possible runout zone of a typical skier-triggered avalanche (max. size 3)
  • Terrain offering the potential for deep burial or serious injury arising from a fall.

Two different avalanche terrain maps covering the whole of Switzerland have been produced. For those who engage in planning tours and assessing conditions in the field, they can provide significant added value.


Map products


Classified avalanche terrain (CAT) map

This map classifies the avalanche territory using red colours for starting zones and blue and yellow colours for runout zones (Fig. 1). What do the colours signify? Red denotes a potential starting zone with a gradient between 30 and 50°. The darker the shade of red, the greater the probability of an avalanche fracture. The potential runout zone is indicated by three blue shades and yellow colouring. The darker the blue, the greater the probability of remote triggering in case of an unfavourably bonded snowpack (typical old snowpack problem). The yellow colouring indicates the possible runout zone of a large (size 3) dry slab avalanche with an average slab thickness of 50 cm.



Avalanche terrain hazard (ATH) map

The avalanche terrain hazard (ATH) map shows how severe or dangerous the terrain is generally, as regards avalanches (Fig. 2). The extent of the hazard is assessed according to a combination of the potential for avalanche fracture and remote triggering on the one hand, and the possible consequences arising from burial or falling on the other. This map layer depicts continuous values with a gradual colour transition from deep red to pale blue. The higher the value, the more dangerous the terrain in respect of avalanches. Unlike the first terrain map, this one does not differentiate between starting and runout zones.



Capabilities and limitations of the maps


Compared with slope angle maps, the new maps offer the advantage of showing where avalanches occur and highlighting areas away from the steep terrain that pose a potential danger. When a user plots a path on the map or selects a route in the field, for instance, he can distinguish between favourable and unfavourable terrain, and assess both the extent of the danger and the likelihood of being struck by an avalanche released higher up the slope. At the same time, the following limitations apply:

  • The maps depict avalanche situations in which small to large (size 3), but not very large, avalanches are to be expected. According to this assumption, terrain without any colouring is relatively safe.
  • Narrow ridges are likewise not coloured in many cases. Places such as these can, however, pose a danger because of other hazards, including cornices or the risk of falling.
  • Terrain in forests classified as “dense” is disregarded, whereas wooded areas regarded as “open” are treated as unforested terrain. No consideration is given to the actual dynamic nature of the forest structure.
  • Differences in aspect and altitude zone, as well as terrain with a gradient of more than 50°, are disregarded.
  • Finally, the maps depict only the terrain, without giving consideration to the conditions, extent of danger or particular avalanche problem; in this respect they are static and disregard the current avalanche danger situation.


Modelling of the avalanche terrain


A variety of methods were developed to describe the potential avalanche terrain types outlined below on the basis of a geographic information system (GIS). The selected data resource was the Swisstopo digital terrain model SwissAlti3D, which has a resolution of 5 metres.

  • Potential starting zones
    The terrain features that typically exist in an avalanche fracture zone were established by analysing the slope angle and topography of more than 5,000 hand-mapped avalanches.
  • Typical terrain for remote triggering 
    Areas where remote triggering is likely to occur were determined statistically by referencing the known distance from the trigger site to the fracture zone of remotely triggered avalanches giving rise to physical injury.
  • Possible runout zones of typical skier-triggered avalanches
    The extent of the avalanche runout zone was calculated with the avalanche simulation model RAMMS::EXTENDED. The maximum size of the simulated avalanches was restricted to “large” (size 3) and a slab thickness of 50 cm.
  • Possible consequences arising from burial or falling
    The potential for persons caught in an avalanche being buried deep in the snow was calculated from the avalanche simulations. In addition, a fall model was developed to estimate the potential for serious injury as a consequence of falling.

The methods and findings that emerged allowed the avalanche terrain to be mapped automatically, largely using data and models. Validation based on avalanche accidents and by experts has shown that the accuracy of the mapping is good. These new maps provide a sound basis for further developments, such as the automatic recognition and assessment of cruxes, supporting route planning, or avalanche danger mapping in real time.


Where are the maps available?