Hiking difficulty (i.e. sac_scale)

I’m interested in mapping off-trail hiking paths which may vary in difficulty from just walking to exposed scrambling. My dilemma is that 1) all ways tagged as highway=path are symbolized the same in most tile layers regardless of difficulty, and 2) the currently accepted tagging scheme ‘sac_scale=’ is based on the Swiss system.

With regard rendering, I’m concerned that mapping a difficult scramble as highway=path will misguide many hikers into dangerous terrain or simply ruin their hike when they don’t find an established trail where they expected one to be. For example, OSM used to have a path leading up the Hornli Ridge to the summit of the Matterhorn, which is much more difficult than a walk. It would make sense for most tile layers to either exclude difficult paths or symbolize them differently, but I don’t know of a single one that does. So is it better to omit difficult paths or is it better to map them regardless of rendering problems?

With regard to the tagging scheme, there is already a sac_scale tag which describes the difficulty of a path. However, this is based on the Swiss difficulty scale which is not used in the United states or many other parts of the world. While it would be beneficial to have a global system, the local grading scales for different regions are already very well established. Using a scale from another region is more likely to cause confusion, and it may lead contributors to exclude a difficulty tag rather than attempt to translate from the system they known into a foreign one. I think a tagging scheme which allows the use of multiple scales would be more appropriate. For example, a class 3 scramble in the Yosemite Decimal System (YDS) could be tagged as “difficulty_scale:yds=3” and/or “difficulty_scale:sac=T5” rather than “sac_scale=demanding_alpine_hiking”. Being a relatively inexperienced OSM user I’m uncertain of whether this idea would garner much support in a formal proposal, and I’m looking for input from the community.

Don’t worry about the rendering done by the generic default style at http://www.openstreetmap.org/ Anyone actually using that as a hiking/climbing map would have rocks in their head. For that use you will want to use a renderer that knows about adding contour lines and some reasonable grid system for backcountry navigation.

I haven’t actually checked it, but based on the rendering file for Osmand I think that software renders based on the SAC scale. Other renderers may do so as well.

As far as altering the actual tagging system, that should be discussed on the tagging mail list. But be aware that most (all?) of those discussions involve a lot of “bike shedding” (a term I only learned about because of that mail list), so it will probably be slow going.

For what it is worth, the Sierra Club/Yosemite decimal system makes sense to me as that is the one I learned first. So it would not bother me if it were added to the mix from the point of view of a user. From the point of view of a person maintaining a private map rendering style sheet, it is bothersome to have multiple ways to tag the same feature.

Yes there are numerous inappropriate climbing routes which are marked as paths on OSM. For instance standard routes for Mont Blanc are shown as paths: these not only are not paths, require good alpine experience (including climbing & glacier traversal), but are subject to major objective dangers (serac and rock fall). This type of mapping is undoubtedly wrong. I suspect that these should be something like route=climbing, or route=alpine.

In my experience the SAC scale is very usable and in general is designed to be easier to grade routes using fairly objective criteria. The SAC scale is for walkers & scramblers and is not relevant to true rock or alpine climbs, although grades 5 & 6 often represent routes which were formerly graded F or PD in earlier SAC guidebooks. In Britain scrambles are also graded using the British scramble grades from 1-3 (which probably roughly correspond to 4-6 on the SAC scale), so there is no reason for not using a local scale as well. However, my feeling is that the SAC scale is about the best we have for using on a worldwide basis: it certainly transfers fairly easily to other European countries), but it may help to have walked/scrambled a couple of routes which have well-established grades in order to feel confident about transferring the scale to other places.

I personally feel that we lack a rendered output from OSM specifically targeted at walkers/hikers & scramblers. Distinction of path difficulty (& equally important paths with significant objective danger) would be one facet of this. French IGN TOP25 maps have a specific symbology for “passages delicats” which would be one way of doing this.

n76, thanks for your advice I will look into the tagging mailing list though I’m not sure I want to be the one spearheading the effort. I can see how it would be difficult to have different rendering rules for different regions depending on what tags are available (e.g. color path by YDS scale in US and color by SAC in Switzerland).

SK53, I’m glad you mentioned the IGN maps because I happen to have the one for Chamonix and the Mont Blanc massif. I would love to have a rendered tile layer akin to these maps and I just found that MapBox Outdoors offers close to this level of detail. It appears to render paths with different symbology based on the trail_visibility tag, but I’m not certain since I can’t find a legend for the map.
I’m also of the opinion that climbing routes which typically require technical skill and equipment should be tagged differently (e.g. route = climbing) if collected at all. It would seem reasonable to collect these routes for some mountain climbs such as Mont Blanc, but many routes are mostly vertical. It’s challenging to collect and portray a mostly vertical way on a 2D map, especially with routes that start three meters from one another and may cross each other.

For now I’m inclined to collect the difficult hiking paths and do my best to translate to the SAC scale, though I may include a secondary tag in the scale I know. I hope the maintainers of the various tile layers will soon learn that these types of paths are inappropriate for their map or that they need to be symbolized differently. To me it seems better to collect this information and make maps that fit our reality rather than trying to conform reality to fit our maps. The fact is these paths exist and are widely used. Mountain hikers would benefit from a good map which displayed them.

Hello, there’s the tag highway=via_ferrata for climbing routes equipped with fixed cables, etc. But you should be aware that some mappers do remove this tag and tag the climbing route again with highway=path because highway=via_ferrata isn’t rendered on the main map.

I would really like to urge you to continue this practice of at least implementing sac_scale. As a developer of a “world-wide” style myself, I can tell you that implementing and maintaining multiple renderings for what are essentially the same phenomena / objects, just because there are minor regional differences, is a nightmare…

There are already hundreds of objects rendered in some styles, adding “regional” differences would lead to a unprecedented, and most likely totally unmanageable, style expansion.

Of course it is possible to develop a “regional” style on a “regional” tile rendering server, using local tagging schemes, but don’t expect world-wide styles to implement them anytime soon… it is just to difficult.

My ArcGIS renderer in development, already renderers both sac_scale and via_ferrata_scale, which should solve this issue. I have shown rendering examples in this openstreetmap-carto thread:


Ultimately, this ArcGIS renderer will become available publicly, when I see it fit for production use.

I’m glad at least one style developer has taken notice that not everything labeled highway=path is suitable for a casual hiker. Since rendering regional differences is such a challenge perhaps the global tag ‘sac_scale’ and an optional regional tag ‘difficulty:scale=’ is the way to go. Is a global + regional tag system such as this implemented for any other features on OSM?

I like the style you’ve implemented and as an ArcGIS user I may like to try creating my own with your rendering tool when available. I had considered creating a small test map in MapBox symbolizing with the SAC color scale. Since the target audience would be peakbaggers and scramblers I think the additional colorization would be more of a benefit than a detraction, but it’s unlikely I would produce a full global or even regional tile layer.

I am not sure if I understand you right, but I slightly get the feeling you are suggesting to mis-use the sac_scale to insert values of another scale, and then use an undocumented secondary difficulty:scale tag to explain what the values represent?

Besides the undocumented tag, I see two major issues with this approach:

  • You are actually mis-using the sac_scale key for values that are not in sac_scale. This will lead to confusion and may actually put people in jeopardy, as the renderer may then display values that might represent a completely different scale from sac_scale, causing misjudgements by the people using the map.

  • It is unlikely renderers or styles will use the undocumented difficulty:scale tag, or that the information even ends up in the (rendering) database, if people use generic tools without modification.

I would therefor like to encourage you to properly translate your “local” hiking difficulty scale into “sac_scale”, so that the values are actually representative of the key’s significance. This is the only way to not put people in danger by misleading them with inappropriate values. I realize there may not always be an “exact” match, but I am pretty sure it will be doable…

You misinterpreted me.
I intend to use the sac_scale as it’s specified on the wiki page. I’ll do my best translate my local scale (Yosemite Decimal System) into the appropriate sac_scale tag. I’ll include the commonly used YDS rating for a route as the secondary tag even if it will never be used. Perhaps someone with more experience in both scales can then use the additional tag for assigning a more correct sac_scale tag at some point in the future. For example, a route that I know as a 3rd class scramble would be tagged as "sac_scale=alpine_hiking’ and “difficulty:yds=3”.

It seems like there must be other circumstances in which it would be beneficial to incorporate an optional local tag in addition to the commonly used global tag, but I haven’t found an example in the wiki yet.

It might be nice to update the wiki page on the SAC scale with a section giving a table with suggested mappings of YDS into SAC scale values. That give a starting point for other people familiar with the YDS to uniformly tag something with a SAC scale instead of having everyone come up with their own interpretation.

I’ll ask around on hiking forums to see if there is an existing conversion chart or if some individuals with more international experience can help me put one together. I’ve had no luck googling so far.

The only other example I can find of a regional tag usage is “mtb:scale:imba”. In the wiki they recommend using this difficulty scale primarily for mountain bike parks, but it’s often used in the US (and perhaps elsewhere) to tag the difficulty of natural mountain bike paths too. Some of the top mountain biking websites (singletracks.com and mtbproject.com) and trail guides in the US use this grade scale, so it makes sense that it be used in addition to the mtb:scale and mtb:scale:uphill tags. I’m imagining a similar tagging system to this for hiking.

A quick look at the YDS suggests 1 corresponds to SAC 2; and probably YDS 2 is roughly equivalent to SAC 3. Equivalence thereafter is harder to match, although just adding 1 from the YDS might work as a first approximation. The problem with YDS is that in practice it is a climbers scale and not a walker/hiker/scrambler scale, and it is relatively old. The SAC scale is more modern, recognises that walker/hiker/scramblers need a more refined scale and one which is easier to apply objectively.

A good example of the relative change is the Fluela Wisshorn. In the copy of SAC Alpine Guide (from late 1980s/early 1990s) which I own this is graded WS (Wenig Schwerig = PD Peu Difficile, Moderately Difficult), which probably comes out as 5.0 or 5.1. I dont have a later walking guide, but on places like SummitPost and Hikr this is either graded T4+ or T5 on the SAC Scale. It has one harder section (which is more intimidating than it seems).. There is very little which is technically demanding of the ascent, but it becomes seriously exposed close to the crux and the summit: the old alpine grade captured none of this.

It’s well worth having a look at a site like Hikr and see how people have graded stuff in the States: there are several german language accounts of the regular “Cables” route up Half Dome, and I’m sure most really popular routes will have been ascended by folk familiar with the SAC Scale. Once you have a few point of reference it should be relatively straightforward to decide how to assign grades.