Misuse of sac_scale in the Alps

Posted by SK53 on 8 September 2021 in English.

I was intrigued by the photo on the wiki illustrating the hardest level of the Swiss Alpine Club scale for mountain hiking (known as SAC Scale on OSM and tagged with sac_scale). I couldn’t identify the location and wondered by looking for ways with the tag I could do better.

A quick overpass query revealed widespread misuse of the sac_scale tag for true alpine climbing routes which the Swiss Alpine Club grades using a completely different scale.

A few of the more egregious examples:

  • Geant-Rochfort Arete. A classic high-level snow ridge encompassing a couple of 4000ers. Tagged with highway=path and sac_scale:

Aiguille de Rochefort

Description of route on Hikr.

  • Biancograt of Piz Bernina: apparently trail visibility is horrible, which is a bit of a surprise, because the ridge is rather narrow.

Piz Bernina W

  • Normal route for Piz Palu. At least this is what it looks like to me, although it is labelled “Piz Spinas - Cresta SW” and when I’ve been at Diavolezza the route through the ice falls has been somewhat different. Although this route is immensely popular most of it is over a very steep glacier with two major ice falls. The existence of significant objective dangers was used in the 1929 movie Die weiße Hölle vom Piz Palü starring Leni Riefenstahl.

Palü en Diavolezza

The normal route from Diavolezza contours above the crevasse field in the foreground and then passes through the lower ice fall quite close to the rock wall on the left before ascending to the minor col to left of the E summit.

There are numerous other normal ascent routes for significant peaks in the Pennine Alps. It is obvious that the tag has been used as follows:

  • normal ascent routes which will be heavily used in the alpine climbing season. In good weather the route taken by climbing parties will be clear, but once fresh snow falls earlier traces are often effaced. On rock it is still possible to lose the line of a frequently climbed route.
  • as a proxy for accurate Alpine Climbing grades (F/PD/AD/D/TD/ED or L/WS/ZS/S/SS/AS etc)
  • often with a highway=path tag (although this was deleted in the case of the Matterhorn)
  • sometimes with surface or trail_visibility tags to perhaps indicate these are not actually paths.

As normal hikers are extremely unlikely to undertake a difficult_alpine_hiking tour (T6 grade), the immediate impact on OSM users is probably not anything to worry about. In the past I followed a few T5 routes, and they were slightly above my comfort level, so I would never have undertaken a correctly graded T6 on my own. However the difference between T6 and all these routes is much greater than that between T5 and T6. They require much more equipment and very different techniques: safe glacier travel, crevasse rescue, climbing at UIAA III, etc).

Elsewhere in the world there have been problems with people assuming a path on OSM represents something within their capabilities. We have a responsibility to ensure the data within OSM does not exacerbate such issues.

Location: Pontresina, Maloja, Grisons, 7504, Switzerland


Comment from Heather Leson on 12 September 2021 at 16:40

Thanks for sharing this. I love learning about Swiss and OSM.


Comment from Hungerburg on 19 September 2021 at 21:00

Let me first point at a misunderstanding in the first paragraph: That there are different scales, a hiking scale and a climbing scale, does not mean, that one starts, where the other ends. It is mentioned in the SAC document, that T6-hiking may comprise WS-climbing, but without the possibilty of securing by rope. The grading document further says, that it is meant to describe the difficulty in optimal conditions, i.e. fine wheather, on dry ground, suitable snow-cover, etc.

This is not to say, that there are trails in the OSM data, that are beyond SAC difficult alpine hiking, and consequently, must not be mapped as a path. So ZS climbing is out of scope, as well as UIAAIII. I find it interesting, that by omitting the “hw=path” tag, the trails can be kept, but hidden from display in OSM-Carto. Is there a tag, that may show them on special “climbing” maps?

SwissTopo has an overlay at that shows all the signed hiking trails. You will get to see the very nice hill-shading in close zoom :) You will notice, that there are not that many blue ones. I looked up “Niesengrat” from the search box, because this is mentioned as a sample for T6 difficulty. There is no trail shown in the Swisstopo overlay to the Fromberghore. That may explain, why there are so few blue trails to be seen. I guess, that is because the demanding and difficult alpine trails are not signed.

Comment from SK53 on 24 November 2021 at 10:16

@Hungerberg: for some reason I missed your comment, which is very interesting.

There is no misunderstanding, although I may not have phrased it well. I am fully aware of the different scales issued by the SAC and the potential for overlap. My SAC guides to parts of Graubunden date from the 1980s, and many routes which are now graded with the “SAC Scale” were previously given Alpine grades (e.g., Fluela Wisshorn Nordgrat was WS and is now T5), although other routes were given grades in the earlier 3 grade scheme which I found fairly useless for identifying suitable routes. In fact when I lived in Switzerland the new hiking scale was only just appearing in publications of the SAC.

Marked blue-white-blue hiking trails are, in my experience, rather limited, at least in Graubunden. There are a couple I’m aware of in the National Park (Piz Quattervals, Fuorcla Val Sassa), plus Piz Ot and Piz Julier in the Oberengadin. The path from S-charl up towards the Fuorcla da Rims is so marked on the map, but I don’t recall any paint markings on the ground (this latter path has changed dramatically in the past 40 years — the older SAC guide books, with black covers, describe it as a glacier crossing). Uri seems to have a much more extensive network of blue-white-blue routes.

Routes to many summits are partially pathless, although equally there may be very visible, but unsigned paths, such as Piz Lischana, although SwissTopo does not show a path on the summit cone. Paths with faint visibility may be present close to the summit (Piz Minschun), but don’t form a continuous line to follow.

Unfortunately I dont have to hand books with a good description of tours graded with the SAC scale (e.g., Freie Sicht aufs Gipfelmeer) which would be the place to start with selecting other examples.

Comment from Hungerburg on 26 November 2021 at 23:29

Hello SK, I am not the least that familiar with Swiss customs as you are, being from a neighboring country, where trails are graded like skiing pistes: blue, red and black. I like it, that openstreetmap has all the paths, that disappear from commercial maps, with each new edition the more. Mostly, they are not fashionable either, so one can hike them in splendid isolation, so to say. I sometimes delete ones, that are obviously wishful thinking or plain misinterpretations of the aerial. I also like to remove sac_scale tags from ‘informal’ paths, not the least, because those tags make them more prominent on OSM derived hiking maps ;) Actually, I am less concerned about dangerous trails being mapped, than about users, who delete paths, because they are dangerous in their mind or give ridiculous sac_scale values, just to make graphhopper not route there. When a hiking accident makes it into the news, my first is, to look what OSM has there. Over the years, I learned of a prominent case, where the “Bergretttung” had to engage for at least two times, where nothing was in OSM, while google maps was the “Navi”. I learned of fatalities, among many, on paths, that are in the official state map too and once on a path, that is a bit en vogue, got covered by many blog articles and is in all the basic hiking guides. The last one surely classifies as T6, in the meaning, WS climbing, no possibilities for securing. The entry is two to three hours uphill walk from the bus stop or car parking. So I do not consider this a danger to the public ;)

Comment from SK53 on 27 November 2021 at 15:16

@Hungerburg: I’m at least also familiar with the ‘ski’ style grading. I have quite an extensive collection of Rother Bergwanderfuehrer where the careful selection of routes and the blue-red-black convention suited me just fine (the Fluela Wisshorn is, needless to say, black). These guides proved much more useful than the SAC guides at the time, in fact I pretty much relied on them, but I think SAC scale has greatly improved things.

I find your comments about a different misuse of the tag intriguing too, and not something I had specifically considered. Equally, in the UK I don’t think we have the same issues with apps like Komoot, although it is widely used, the number of trails which justify anything from T3 upwards is quite small. There have been discussions for a long time on the Carto-CSS github repository to render harder trails in a different manner, but these have not come to anything.

I’ve just checked a route in Scotland where a friend lost his life two years ago, and it is on OSM as a path, but with a reasonably appropriate grading. He was an experienced hill-walker: I don’t know the exact circumstances, but I fear that he may have encountered late Spring snow.

Comment from Hungerburg on 29 November 2021 at 23:39

Hello SK: Early autumn, I was in the Karwendel, paying visit to friends that herd some cows there, with a hotel-hut close to the Alm; I had dinner in the touristic hut. I like to ask people, to let me have a glimpse on their Navi. I found does a good job at rendering “trail_visibility” - A value of “no” makes a path nearly invisible on the map. Of course, thats the one I chose earlier that day. In fact, there was no path, a pleasant hike over grassy slopes up to a summit, that once had a bicycle as a cross mark. I was the only one there, caravans went to the prominent summits that day. After dinner, the host advised people, to go to the second prominent summit then, because of snow fields on the most prominent one. The photos he showed, did not show anything difficult. I tried to pith the other summit as truly easy, he just said, 10 per year go there, nothing further. The next morning I went examining another “mysterious” OSM “path”; Again, a very interesting hike, mostly along a pressurized water pipeline; I had to delete a part of this path, because it created a routable connection, even though no signs on the ground showed actual use as such shortcut there. On the retour, I went the touristic path, which features some assisted passages; The next day, a person fell to death on this one. I remember, reminding myself to be careful in the very location, but I never had thought, that it was about life or death. In English, its called accident, in German its called Unfall, both means, German maybe a bit more concise, what one is not expecting to see happening. Otherwise, it probably would not happen.

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