seamark:bridge:clearance_height for bridges over waterways

[Tomagori]

General

This information is highly relevant to people planning boating trips, especially when navigating canals. The relevant information is often sign-posted close to the bridge, so should be easy to obtain and enter into StreetComplete.

Affected tag(s) to be modified/added: seamark:bridge:clearance_height, seamark:bridge:clearance_height_closed, seamark:bridge:clearance_height_open.
Question asked: What's the vertical clearance for boats/ships below this bridge?

When a bridge=movable or equivalent is involved: What's the vertical clearance for boats/ships below this bridge when the bridge is closed? and What's the vertical clearance for boats/ships below this bridge when the bridge is open?.

EDIT: just noticed "closed" and "open" might invite misunderstandings. When a bridge is closed for street traffic, it can be considered open for marine traffic, so maybe it should rather be "when closed for street traffic" or something similar.

Checklist

Checklist for quest suggestions (see guidelines):

• 🚧 To be added tag is established and has a useful purpose
• 🤔 Any answer the user can give must have an equivalent tagging (Quest should not reappear to other users when solved by one)
• 🐿️ Easily answerable by everyone from the outside but a survey is necessary
• 💤 Not an overwhelming percentage of elements have the same answer (No spam)
• 🕓 Applies to a reasonable number of elements (Worth the effort)

Ideas for implementation

This could more or less be the exact same thing that already exists for bridges with streets underneath them, only this time for waterways that run under those bridges.

Element selection:
EDIT: It is of note that these seamark: tags are most often applied to a single node on the waterway that's just below the bridge. Ideally, a node tagged with seamark:type=bridge should already exist where this quest is relevant. In case it is not, this node may have to be created by StreetComplete first. When then adding the clearance value, seamark:type=bridge should be added to that node as well.

Waterways under bridges could be identified by looking for ways and areas tagged with natural=water and/or waterway=*, with anything but "no" tagged for the boat=* and ship=* (or subtype) keys, that intersect with ways or areas tagged with bridge=*.

seamark:bridge:clearance_height_closed and seamark:bridge:clearance_height_open would only have to be checked if bridge=* is set to movable. Also check out the values for seamark:bridge:category=* that denote movable bridges of various types that should be checked for as well probably.

Proposed GUI:
Pretty much nearly the same thing that already exists for bridges, just with a boat instead of a car in the image.

[matkoniecz]

What would be tagged if such sign is not present?

[Tomagori]

Good question, maybe something similar to how it is with streets, asking if there is at least X amount of clearance, with X depending on whether the waterway is only tagged with boat=* or ship=*. However, it's possible we'd be talking about sizes too hard to gauge with eyes only, and requirements for boats or ships might differ too much to allow for universally applicable answers, so maybe "there is no sign" should just hide the quest or create a fixme= measure clearance.

[smichel17]

The problem is that whatever is tagged for "There is no sign" must hide the quest for everyone, not just the person who selected the option.

• 🤔 Any answer the user can give must have an equivalent tagging (Quest should not reappear to other users when solved by one)

[Tomagori]

In this case, using no_indications would seem appropriate. The current wiki article doesn't reference non-numerical values, but this seems well within the logic of the tag.

[Tomagori]

To add to this idea: Looking at the seamark tags, it seems they're being neglected a fair bit. When setting seamark:bridge:clearance_height, seamark:type=bridge should be added to the same node if it's not already there.

[westnordost]

A user of this app is usually on foot. It'll often not be possible to see what's on the sign that is facing the river or body of water.

[Tomagori]

Speaking only from personal experience, these signs are very visible to people passing by these bridges on adjacent paths. Especially in areas with canals, you'll often find cycleways or footpaths next to them, so I don't think this should be an issue. I actually only had this idea because I noticed a bunch of these signs on a jog one day.

[westnordost]

I think this only works if there is a footway alongside the river. In Hamburg, most bridges are like this:
https://www.google.de/maps/@53.5816545,10.0102479,3a,75y,112.89h,71.66t/data=!3m6!1e1!3m4!1ssWE15hFKjloJhaXe6iOj8Q!2e0!7i13312!8i6656

In your area, it might be different, but the simple fact that in other areas, the quest may always not be answerable is problematic because

🐿️ Easily answerable by everyone from the outside but a survey is necessary

[Tomagori]

In my case, it was actually just outside of Hamburg, coincidentally :D I see what you mean. I'd still say that in those cases, you can look over the railing and probably spot the sign if it's there, I've even seen clearance values edged into the ground on top of the bridge (close to the railing), so I think it would still be doable. But I understand if this kills it for you.

[smichel17]

A higher-effort solution could filter to only situations with a footway alongside, reversing the logic from the cycleway/sidewalk quest.
I can't say if that's worth the effort.

[Tomagori]

I'd say that, as it breathes life into a neglected but highly useful tag and considering the information is out there, (often) visible, with too few people taking the time to actually enter it, SC could make a very valuable contribution here.

[westnordost]

Alright, maybe. However:

• What tag exactly to use if there is no sign?
• should only be asked for bridges crossing rivers with boat=yes
• river height varies during the year. How is this taken into account?

[Tomagori]

I'd suggest:

• If there is no sign, tag no_indications
• I'd not only restrict it to yes, but go for anything except no. If SC usually doesn't tag private, then exclude that, but I'd say there's value in adding the quest for permissive and especially for designated too. In addition to the boat=* key, I'd consider the cases where boat=* is missing, but one of the more specific subtags is present (motorboat=*, sailboat=*, canoe=*). Arguably, ship=* should be covered as well. While the clearance tag may be moot for large bridges like the Øresund Bridge, the various inland commercial shipping routes throughout continental Europe for example provide ample use cases for the tag (and it's already in use there). So if a bridge crosses the Danube where there's no boat=* but only a ship=* tag, the quest makes sense there too. Therefore, to avoid inaccuracies and impossible quests, maybe excessively long bridges could be excluded (anything over like 300 meters), but the ship=* tag could generally be covered as well.
• At least in Germany, clearance values seem to be calculated from a variance-adjusted point. As the signs (at least the ones I know) all only mention one value (though some bridges occasionally have gauges, either in addition or exclusively), this shouldn't be an issue for mappers in the field. When there is no clear sign, no_indications should be used, as the seamark tag doesn't accommodate variability as far as I know.

[matkoniecz]

If there is no sign, tag no_indications

Is it tagging scheme that is in use? Seems not from looking at https://taginfo.openstreetmap.org/keys/seamark%3Abridge%3Aclearance_height#values

[Tomagori]

Good point! I suppose unknown would work as well, no_indications, as it is used for maxheight on streets, might be more accurate, however.

[matkoniecz]

I also would prefer no_indications but it should be validated on some general forum before using in SC (to be done if it is a remaining step).

In general I worry about problem of bridges with such signage that are surveyable only from water - despite carrying footway/road.

[Tomagori]

True, I'll open a thread on the wiki if this moves forward. Maybe a differentiation would be sensible. If I can't see a sign, but I can't see the bridge from the side either, I tag "unknown", if I see the side and there's no sign, I tag "no_indications". But I'd have to check back with people on the wiki.

[matkoniecz]

If you look for feedback tagging mailing list may be far better - on OSM Wiki you likely will get just reply from me :)

[westnordost]

A new issue can be reopened when the tagging issues have been resolved (by using the usual channels)