Trail_visibility explained

Trail_visibility was introduced as a companion to sac_scale key more then ten years ago. As today got mentioned here, Use of sac_scale in non-mountainous areas by @rhhs the link is no longer prominently showing in the documentation.

As a matter of fact, the Mountain Hiking Scale developed by the SAC/CAS does not have that concept of trail_visibilty - this is a fully original OSM notion. Let me explain: The SAC scale talks about routes over paths and pathless terrain. The higher the grade, the less path, the more pathless terrain will be travelled along the graded route - exemplary ASCII art:

=============== T1 Hiking
--------------- T2 Mountain Hiking
----  ---- ---- T3 Demanding Mountain Hiking
---   ---  ---- T4 Alpine Hiking
---  -  - -  -- T5 Demanding Alpine Hiking
---   -     -   T6 Difficult Alpine Hiking

= means a trail well cleared (Weg gut gebahnt T1)
- means a continuous trail (Durchgehendes Trassee T2)
A blank means pathless terrain (intermittent Trassee?)

The higher the grade, the less path there is. But not in the sense of a gradient in visibility, but in the binary percentage of where there is a path, or where there is no path along the route.

That is why the documentation no longer mentions SAC scale.


I assume (I’m a more recent participant in tagging discussions) that the main reason to split off trail_visibility from the hiking difficulty grading system we use here is that in the original SAC scale, they were assumed to change in parallel, i.e. a more difficult trail is also assumed to be a less visible trail. However mappers found many examples of trails that were easy but not very visible (a trail across a meadow, for instance, is often easy but difficult to see) or difficult but easy to see (a via ferrata for instance).
The sac_scale key we now use for hiking difficulty still is a combination of technical difficulty (how much you have to lift your feet, whether you need your hands, etc.) and danger (exposure, risk of falling). @erutan proposed a few months ago to split these as well (here) but this was not received well.

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You might be recent, but I find your thoughts on this enlightening. I love to ramble and to scramble, I always considered the split as something only people that are completely foreign to subject matter can come up with; As if you could do a T6 scramble in a gym.

To echo @erutan: trail_technicality would much better convey the other part of the split. As of today’s sac_scale, if there is no trail, it cannot be hiking, because hiking requires a well-cleared, a.k.a Well developed, signposted and marked path.

That’s not how I interpret that phrase. For me, “well cleared” means “free from obstacles” (that could twist your ankle if you’re not wearing hiking shoes). Whether the path is free from obstacles because some human removed them or because nature never put them there is irrelevant, I think. Maybe we should replace “well cleared” by “free from obstacles” in the sac_scale wiki?
Whether it’s signposted and marked is part of trail_visibility, not sac_scale, I think.

Yeah I’d agree with that. If terrain naturally doesn’t need any clearing and it’s as simple to hike on as terrain that had to be cleared it basically the same thing (visibility is another issue, tread, markers, etc).

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Mind you, the scale by the SAC says “gut gebahnt”, canonically (Oxford dictionary, DeepL) this translates to “well paved”. And that is not about markers at all, it is just about the appearance of the trail itself.

Now, removing that term “gut gebahnt|well cleard,pave” from the wiki documentation of sac_scale looks plausible, but that would leave us with little means to distinguish hiking from mountain_hiking?

You are welcome to develop a scale that does not carry the burden of another scale! I already suggested trail_technicality - corresponding trail_visibility - Though I’d prefer something that not only is useful for experienced mountaineers but can be of use for average city dwellers that only happen to leave the funicular station that a bureaux trapped them in.

PS: I started a poll in the German forum what separates hiking from mountain hiking, the community is split in a way, that is not enough to win approval of either side.

I’m confused what trail_visibility has to do with the sac_scale. Climbing scores are an rating that represents the average difficulty involved in completing a particular trail. This assumes that the trail in question even has a SAC score.
The trail_visibility specifically indicates how well-defined the trail path is. No or 0 means the trail is effectively indistinct from the surrounding area. On the other end of the scale, trail is painfully obvious. You will likely notice it even if you were not trying.

Trail_visibility says nothing else about the trail. Other properties such as difficultly, surface, width are tracked independently.

Remember that just because a trail is easy to find doesn’t make it easily to traverse.

I did not propose to remove it, I proposed to rename it to “free from obstacles”. In this way it is clear that it refers to the difficulty of hiking on it, not to any visual aspect of the trail. The hiking value would then mean that there are no obstacles on the path, so there is no risk of straining your ankle on an obstacle and you don’t need to wear hiking shoes to prevent that. If there are such obstacles, you’d better wear high hiking shoes and the trail should be tagged mountain_hiking.

(from the discussion in German) Our scale was originally based on the SAC scale, but this is no longer the case so we are free to do what we like with it in OSM no matter what SAC is doing in the meantime.

To avoid any more confusion, it may be a good idea to rename the key from sac_scale to hiking_difficulty to make that clear. The key sac_scale could then be redefined and used to tag trails with their hiking difficulty as determined by the SAC (making it 100% verifiable). It would probably only be applied to trails in Switzerland.

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Why not just use smoothness? A valley full of boulders is likely just as easy to find but more difficult to navigate.

It might even make more sense add a barrier tag to a segment to describe the type and relative of size of larger obstacles. Such as barrier=block in case of a trail obstructed by limestone blocks from a nearby ruins.

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Thanks for your reply: it made me realise that “obstacles” is not the right word. I mean free of things like tree roots, stones and other unevenesses that you might trip over, strain your ankle on (so you will want to wear hiking boots to protect your ankles) or otherwise hurt yourself, and that force you to watch your step. On a trail with sac_scale=hiking and trail_visibility=excellent, you can discuss politics, religion or the meaning of life :grinning: with your hiking partner without needing to pay attention to the path and without risk of tripping over a tree root or loosing the path.

What would be a good word for “things that you can strain your ankle on”?

That’s not how I interpret that phrase. For me, “well cleared” means “free from obstacles” (that could twist your ankle if you’re not wearing hiking shoes).

I think according to the context, well cleared doesn’t guarantee there aren’t any obstacles that can twist your ankle, it means vegetation has been removed, as well as most obstacles (compared to “normal”), but in alpine regions the typical (normal) situation is the requirement for hiking shoes even after it is “well cleared”

Whether the path is free from obstacles because some human removed them or because nature never put them there is irrelevant, I think.

“cleared” implies they have been there and have been removed, otherwise it would be “clear”

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The majority of single track trails in the Sierra Nevada are neither free of anything which you can trip over, nor proper T2 where you are moving over a constant uneven surface. Just because there’s some obstacles in the trail doesn’t mean that it’s hard to see where it goes or that you need boots - many times you can walk around obstacles on a trail. It’s also pretty common for people to hike off trail in rugged terrain with low top approach shoes. My partner will carry a heavy pack on T2-T4 terrain with minimalist trail runners.

I also read well cleared as that vegetation will be cut back and branches removed from the trail, not that every single rock and root has been taken out of the path. In order to be visible it just needs to be easily distinguishable from the ground around it - a trail can have a lot of roots on a dirt surface and still be clearly distinct from thick vegetation on either side.

None of the photos on Key:trail_visibility - OpenStreetMap Wiki show trails completely free of obstacles.

This sounds like my proposed hiking_technique. I’ve been thinking about the obstacle issue I outlined above over the summer, and someone else in the thread made a point about it as well.

I took a brief stab at this earlier by looking at a variety of international bodies that create trails, feedback from other threads, and my own experiences.

It’s a very rough draft and needs some editing and condensing, but I think it does a good job of establishing a truly “excellent” excellent and not getting bogged down in a lot of confusing overlapping lower ends of the scale.

In regards to the earlier comment on exposure, I’d be fine with just dropping exposure from difficulty but not creating a new key and just using hazard=* nodes though there’s some things to sort out there.

@Hungerburg many months ago I mentioned some photos of a trail that fades - here two sides of the same trail at a meadow where it loses definition (not enough foot traffic to overcome growth)

The rest of the trail is always solidly visible, so I wouldn’t downgrade the entire way, but it also seems annoying to split into a new way with trail_visibility=no for ~150m of easily navigable open terrain.

If I was to follow this trail guiding myself with a map, I’d be glad to see the trail going fainted on the map on this section, so I would trust I’ll find it again across the meadow.


There’s a LNT take that there shouldn’t be a path shown there, as concentrated use could lead to rutting in the meadow which would impact drainage, could kill off areas of grass, etc. It’s unlikely to happen in this case given where it is and the unlikeliness of it becoming very popular but it’s definitely not a case where you need concentrated use to reduce braiding.

For routing purposes a way/segment with trail_visbility=no makes sense.

Well, that is the single most reason, why the tag trail_visibility=no exists at all: To map easy hiking sections over pathless terrain. (From 2012? ML discussions on why that key got split from sac_scale.)

If there was a gap there instead, you would just as well find the continuation. But you could not let routing software guide you there.

Yeah that’s the only legitimate use of it I can think of, otherwise it’s a pathless route masquerading as a path.

Ideally a map renderer wouldn’t display it but routing would work - it’d be better if people can see either side of where the trail exists and make their own way across it to spread out damage to the alpine meadow since the trail has regenerated. Since there’s a path there now anyways and there won’t be many people on it I’m probably just overthinking it. :slight_smile:

Alpine meadows are a thing of the past. Alpine pastures still exist. Sounds like a play on words? A marketing person called them something, lots of other places look with envy at.

It’s more : following the dots, at least on the map, is a small pat on the shoulder.