[Tagging] Bridges redux

Christopher Hoess cahoess at gmail.com
Wed May 8 16:20:55 UTC 2013

Greetings to the list,

I've started revising my bridge tagging proposal
based on the comments received in the last go-round in January. There
are three specific questions I'd like some feedback on before I submit
a full RFC again.

As preface, I should note that I feel it is important to create at
least one key in addition to "bridge". Trying to fit all of the
proposed values in one key will inevitably lead to conflicts: how do
we represent a beam bridge that is also covered? A viaduct that is
also an arch bridge? a cantilever made of truss spans? The "railway"
key has problems caused by the all-in-one approach: how do you
represent a disused narrow-gauge heritage railway? I'm not going to
float a bridge proposal that sets us up for similar problems.

That said, the first of my questions is "How should we divide these
values among keys"? My proposal basically follows the dichotomy Martin
Koppenhoefer laid out here
one set of tags for "typology" (originally proposed by me under
"bridge"), and one set for "structure" (originally proposed by me
under "bridge_type"). I'm in favor of placing the "structure"
classifications under "bridge:structure" to make it clear what's being
classified; there are about 550 occurrences of "bridge_type" at
present, most of which can go to "bridge:structure". The bigger
question is whether the "typological" values (covered bridges,
viaducts, trestle) should stay under "bridge" or move to a
"bridge:type" key. The main disadvantage of this move is that there
are a large number of existing "bridge=viaduct" instances (about
27,000). The advantage of putting typology under "bridge:type" is that
it reduces the number of types a renderer or downstream consumer has
to be aware of. All ways tagged with "bridge=yes; bridge:type=..."
would be rendered properly as bridges without changes to the current
renderers, as would future additions or removals of types. If we
consider this, we might also consider whether "movable" should be
under "bridge" or "bridge:type". Placing it in the former would mean
patching current renderers (e.g., Mapnik), but placing it in the
latter makes specific movable bridge types (e.g., bascule, swing)
rather wordy to tag, with three separate keys.

At present, I tend to favor placing the typology under "bridge:type"
except for "bridge=movable" but I would like further opinions and

My second question is whether culverts should be included in this
proposal. I included values for culverts in the "structure"
classification to maintain compatibility with the Humanitarian Data
Model <http://wiki.openstreetmap.org/wiki/Humanitarian_OSM_Tags/Humanitarian_Data_Model>,
but tagging culverts on the overhead way (rather than the way that
passes through them) is a very small minority tagging style. JaakkoH,
who's active in Haiti work, suggested dropping it, and I'm inclined to
follow his advice.

My third question is what to do about the "drawbridge" value for the
proposed "bridge:movable" key. NE2 pointed out that this is something
of an attractive nuisance; people tend to use "drawbridge" when they
really mean "bascule". Should we drop this value from the proposal
because of the potential for misuse? Is there another name that can be
applied to those bridges?

Your comment on these three points is much appreciated. Once I feel
like I have a sense of the community's position, I'll revise the
proposal page and put it up for RFC again.


Chris Hoess (choess)

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