15. March 2017    

Digital terrain models and slope angle maps derived from them, can be a very practical aid for planning safe backcountry tours. Using the example of OpenSlopeMap, we'll show you what you need to know about slope inclination on the map and on the spot.


The inclination (steepness) is a decisive criterion for all assessment strategies for avalanche risk, but never the only one, neither on the spot nor in the previous planning. The avalanche danger must never be judged "merely by a major factor (criterion)," stresses the avalanche researcher Werner Munter, who revolutionized the applied avalanche studies with the 3x3 method at the beginning of the 1990s. Since then, with all common methods the avalanche warning level is combined with the slope angle, exposure and other risk-influencing factors.

Slope angle and risk of avalanches

splitboarding safety card

Beginning at 25 degrees, the risk of avalanches increases with increasing steepness until the slope becomes, at about 60 degrees, too steep to accumulate enough snow. A simple method for estimating slope inclination in the terrain is the "pole method". Smartphone apps or some avalanche receivers can do this job even more exactly and easier. As a point of reference: Starting from about 30 degrees one begins to ascend using kickturns.

Depending on the avalanche warning level, however, not only the steepness in the immediate vicinity of the winter sportsman is important! Depending on the avalanche warning level, however, not only the steepness in the immediate vicinity of the winter sportsman is important! While at level 2 the track area should be below 40 degrees, at level 3 already the steepest part (10x10m) of the entire slope must be less than 35 degrees, while at level 4 the entire terrain chamber must not be steeper than 30 degrees in any part.
In practice, slopes and entire terrain chambers very often reach far out of our field of view and anyway, we can measure the slope only at our location. It is therefore all the more important to do a proper route planning using a topographical map.

Calculate slope angle in a paper map

slope triangleWith the help of the contour lines, the elevation difference of two points can be determined in a topographical map. If one also measures the horizontal distance of the points in the map, the slope or inclination can be calculated from these two values (right-angled slope triangle).

Horizontal distance (l):Height difference (h):
Slope angle in degrees (α):Slope in percent:

Appropriate help scales as printed on our safety basics card, can be used to measure slope angles in the map. To do this, place the card on the elevation lines and move the scale until the line spacing matches.

Digital Slope Maps

Much more comfortable than measuring each slope individually in the map, are slope angle maps. Here, the critical slopes can be read directly as a visual interpretation of the contour lines, thus making it easier to identify key points and circumventing possibilities.
Just as for the elevation lines, the informative value depends on the accuracy of the collected data. While previously measured, triangulated and interpolated in the field, surveyors and cartographers were the first on many peaks, today height data is gathered by laser scanning from satellites, airplanes and helicopters down to the centimeter range.
This accuracy is not necessary for the winter sports enthusiast, the snow overrides the landscape and at avalanche risk management we look for distances ranging from ten to several hundred meters. The 10m model is therefore a very suitable compromise, on the basis of which, for example, the Government of Tyrol has already displayed slope angels in its webmaps very early. The data basis is shared by the Land of Tyrol to the general public as part of the Open Government initiative. Very exemplary, because now we are coming to a project which combines the height data of various free sources in a single slope map, the OpenSlopeMap.


"The slope slope map for alpinists, ski- or splitboard tourers and freeriders", made by two peers, Peter and Reini from Bavaria.
In their spare time, the two have prepared and processed the best, freely accessible digital terrain models (DTM) for the Eastern Alps. For this purpose, shadows, elevation lines and slope inclinations were calculated and enriched with data from the OpenStreetMap project. Afterwards, the result was split into several million small tiles, which can then be quickly delivered and displayed, depending on the map and the zoom level

Here, for the sake of illustration, the various phases, from the naked data to the colorful map, have been comprehended and integrated as switchable layers for a tiny section for you to explore.


Specifics of the OpenSlopeMap

The introduction of extended slope gradients between 40° and 50° follows the fact, that in this extremely critical area every additional degree of steepness increases the risk of alpine dangers drastically. The two authors and we also point out once again: The slope inclination alone can never be the only criterion in which you are oriented in avalanche risk management! Apart from this, even well-resolved hillside maps are only partially representing reality.

In addition, it should be noted that the OpenSlopeMap inclinations were generated from differently resolved basic data according to the region - for Austria and South Tyrol in 10m resolution, for Slovenia in 20 and for Bavaria on the basis of a 30 meter DTM.

Now for these regions it makes sense to offer you a small planning tool, that allows you to easily display, track or even freely draw tracks and then export the finished route as a gpx or kml track. Try it yourself:

Launch route planner

We thank the makers of the OpenSlopeMap for this useful tool and we are very pleased to be able to use the map data here on splitboarding.eu. In future, the slope map will be extended to other, alpinistically interesting regions as soon as freely accessible data are available for this purpose. You can directly promote the project on openslopemap.org and generally by supporting OpenData and OpenGovernment initiatives wherever they appear and make sense to you.

OpenSlopeMap Download

For offline use, the map is offered for download free of charge in the formats mbtiles and trekbuddy and thus can be used en route on your smartphone.

However, we always recommend to take an original paper map on the tour as a backup, preferable an alpine club map in the scale of 1:25.000.