[Tagging] Chaos and uncertainty in "bridge"

[Martin Koppenhoefer]

Currently the documented values for "bridge" don't seem to follow any
(single) classification system.
http://wiki.openstreetmap.org/wiki/Key:bridge

Fortunately 98.49% of all actually used values don't have this problem
(they are "yes"), but the rest seems a mess:

1. There is a first classification system using the kind of way going
over it for distinction:

1.1 A road (or railway): "viaduct" (a term that is not really well
defined, especially the distinction between a "bridge" and a "viaduct"
doesn't seem to be clear). This is a bit mixed because besides the
road this seems to be a bridge with several abutments in "small"
distances, whatever "small" is, and "viaduct" seems to be used also in
conjunction with railroads. Not sure but I feel as "viaduct" might
also be a bridge typology (see 3).

1.2 Water: "aqueduct" (suitable for the parts of aqueducts that span
over a void). The definition seems to extend the use to all kind of
aqueducts ("A longer structure for carrying a canal or fresh water.")
while many aqueducts are not spanning over something (they are tubes,
have a solid support like a wall without openings or even are
underground) so they clearly aren't bridges.

This first system doesn't make much sense IMHO, because you can
already see by other tags which kind of way is on top (waterway,
highway, railway), but it is currently the most used.


2. Another classification system on the page is one according to the
structural system employed:
2.1 arch
2.2 pontoon
2.3 suspension

3. And even another system is that of typology:
3.1 bascule
3.2 drawbridge
3.3 humpback
3.4 lift
3.5 swing


"so why is this bad?" one might argue. Well, the problem is that with
this chaos you won't be able to tag all properties (typology,
structural system, carried way) of a bridge, you will instead have to
decide which one to focus on (might also lead to tagging wars).
Another problem is that the lack of systematics makes it difficult to
extent this system with new values, because it is not clear where the
focus is.


I propose to use distinct tags for these properties instead:

1. is not needed IMHO (see above). If the interesting fact for
viaducts are the several "small" spans I'd put this into typology.
2. could be tagged with bridge:structure (or structure or structural_system)
3. could be tagged with bridge:type (or type).



Last but not least I'd like to ask you for comments on 3 new values:
N1. a bridge made of few ropes where you walk on a rope:
http://bauwiki.tugraz.at/pub/Baulexikon/HaengeSeilBrueckeB/Kaiserschild_1.jpg
http://www.gruppenstunden-freizeit-programme.de/ferienlager-freizeiten-erlebnisse/freizeit-bilder/seilbruecke/Piratenlager-05-262.JPG
http://www.bergsteigen.at/pic/d6025434-21f9-4d93-9ce9-42aba5cf00db.jpg

additionally we could tag the amount of ropes (or even more precisely
the amount of "upper" and "lower" ropes)

are these described in English with the term "zip-line"?
http://en.wikipedia.org/wiki/Zip-line


N2. a similar bridge made of ropes, but you walk on planks:
http://bauwiki.tugraz.at/pub/Baulexikon/HaengeSeilBrueckeB/Trift_Bruecke_1.jpg
http://bauwiki.tugraz.at/pub/Baulexikon/HaengeSeilBrueckeB/Tripsdrill.jpg

I guess this would be a "simple_suspension_bridge"


N3. A "Cable-stayed_bridge" (the absence of this value makes it
probable that most of these might currently be tagged as suspension
bridges or not classified at all). The difference from a suspension
bridge is that the cables are directly attached to the towers / pylons
instead of to another cable, see here:
http://en.wikipedia.org/wiki/Cable_stayed_bridge

cheers,
Martin