[Tagging] siphon underpass

[Tod Fitch]

> On Jun 8, 2017, at 6:40 AM, Volker Schmidt <voschix at gmail.com> wrote:
> 
> I am looking into how to tag a frequent feature in my area, i.e. a siphon underpass, known in Italian as "botte a sifone" or "botte sifone" and in French as "pont siphon". This is a non.connecting waterway crossing where the lower waterway passes through a U-shaped siphon. The bottom part of the U is a tunnel that is lower as the normal level of the waterway.
> Up to now I have mapped them as tunnel= yes, and more recently as tunnel=culvert, but they are so frequent, and different from a culvert, that I would like to start tagging them with tunnel=siphon_underpass.
> They are also used to pass non-navigable waterways under roads and other obstacles.
> There are hundreds or more in Northern Italy and I presume in other flat parts of the world where there are many artificial waterways. I remember to have seen them on drinking_water canals in California, but don't remember where.
> 
> I could not find any tagging schemes for this in OSM, but I may have missed them in ignorance of the proper technical terms.
> 
> A locally famous example dates from the 16. century: il ponte canale Montaigne:
> http://osm.org/go/0IA2uooQI?m= <http://osm.org/go/0IA2uooQI?m=>


FWIW, it sounds what is being described is not a siphon as I understand the term. It might be called a “inverted siphon”. Which, as wikipedia notes, is not really a siphon [1].

While there are specific cases where the average mapper might be able to determine that the covered section of a waterway is an inverted siphon, it seems to me that this won’t be possible in the general case. After all, the feature is usually buried.

[1] https://en.wikipedia.org/wiki/Siphon#Inverted_siphon
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