[Tagging] waterway=derelict_canal

[Andy Townsend]

On 23/08/2015 16:32, Mateusz Konieczny wrote:
> To people using this tag: please, update and clarify documentation of
> this tag on OSM wiki. For example it is even unclear whatever it should
> be expected that it can be applied to canals no longer filled with
> water.
>
> "used for transportation, waterpower, or irrigation" so filled by water
> seems intended as implied - but canal that is drained and converted
> into cycleway also fits "transportation".

I think that that part of the wiki's fairly clear - you've selectively 
quoted the second half of a sentence there, the part that means 
"canal".  The tricky bit is what "derelict" means...

> On the other hand name (at least for me) implies canal where earthwork
> is present but is no longer filled with water.

Let's assume to start with that we're talking about things that are 
still a feature of the landscape (and leave the "what if they're not" 
discussion to the various "railway" threads).  I've tended to use it on 
those bits of canal that can't be used for their former purpose, so 
everything from:

http://www.openstreetmap.org/way/48784326

(to which I added "description=Water in places, but some trees growing 
in it - disused") through to:

http://www.openstreetmap.org/way/103431234

("note=Here the footpath seems to be along the old elevated canal bed")

To my mind a waterway=derelict_canal is not just a waterway=canal that 
happens to be disused - it's one where the infrastructure has 
deteriorated significantly.  So for example, this section of Cromford Canal:

http://www.openstreetmap.org/way/39551657

is largely disused as a canal (although the charity that's restoring it 
does run trips up and down the top end), but it still has water in it, 
and still looks to the casual observer like a canal, so I mapped it as such.

FWIW I agree completely with Chris about the problems with "foo=bar + 
disused=yes".  It's a form of tagging that's really best avoided. As an 
aside, when I tried to represent "dead pubs" on a map I ended up doing this:

https://github.com/SomeoneElseOSM/SomeoneElse-style/blob/master/style.lua#L368

That's not so bad in lua, but imagine writing "... and not disused=yes" 
into every cartocss rule!

Cheers,

Andy