[OSM-talk] Mapping canals

Richard Fairhurst richard at systemeD.net
Tue Jan 22 11:43:25 GMT 2008


A few more comments, and like Stephen, I've not commented on those  
where I agree. Generally we should make sure that tags are applicable  
to all navigable waterways, so river navigations as well as canals.

Gervase Markham wrote:
> Narrow sections are denoted by maxwidth. One narrowboat (just over 7  
> feet) is given as 2.5m. Two boats is 5m. It's not necessary to mark  
> a two-boat width restriction for bridge holes, which are implied  
> narrow.

So let's start with the trickiest bit of the whole thing. :)

maxwidth and maxlength are the maximum dimensions for the waterway,  
not the physical dimensions of the channel - because generally,  
maximum dimensions are set by locks and bridges, not by channel size.  
(Some long lock-free sections do have a maxlength dictated by bends:  
the Wolverhampton Level and the Bridgewater spring to mind.)

For the Trent & Mersey Canal (Burton-Middlewich), this would be  
maxlength=72ft, maxwidth=7ft. For the River Rhône it would be... a bit  
bigger.

If you have correctly tagged a waterway with maxlength and maxwidth,  
then, there is no need to tag bridges, and lock nodes, with maxlength  
and maxwidth: this is implied.

maxwidth is therefore the maximum width of a boat to pass along this  
canal (just as maxheight on a road is the maximum height that a truck  
can be). The channel will typically be more than twice this width. So  
we need a way of tagging the exceptions - places where only one boat  
can proceed at once. Something simple like "narrows=yes" would work,  
or maybe "channel_width=7ft".

(The rest of the comments are easier!)

> "boat=private" is used for private parts of the canal.

In Britain, at least, all canals are essentially private. There is no  
public right of navigation on canals. I see what you're saying, but  
the terminology will need to be made explicit on the wiki page.

> Locks
> -----
> The wiki page makes some good points, but I suggest we have *both*  
> "waterway=lock_gate" on nodes (useful for large locks, optional for  
> small ones) and "lock=yes" on canal/river ways (compulsory, easy to  
> render).

I still strongly recommend that lock=yes is optionally applicable to  
nodes (and whatever the wiki decides, that's how I'll tag). It will  
make editing _so_ much easier than tagging up countless little ways up  
the Tardebigge Flight would, and there is no loss of meaning.

(In fact, tagging a lock as a way could be misleading. For a UK 70ft  
lock, the length of the way will typically not be the length of the  
lock, unless your GPS is really accurate. Elsewhere, locks are  
generally bigger, and the way approach makes more sense.)

The same tags can still apply to the node, and the direction is  
inherited from that of the canal.

> A flight of locks with a unifying name (e.g. "Hatton Locks") is  
> denoted with a node placed in an appropriately central position with  
> new tag value "place=lock_flight" and "name=<name>".

Agree with Socks that a relation would be good for this.

> Moorings
> --------
> Mooring info should be attached to the relevant stretch of towpath,  
> or to a new dedicated way on the opposite side, for the rare offside  
> moorings.

Mooring is on the water so I'd submit it makes more sense to tag the  
waterway, not the towpath - not least for routing purposes. The side  
could be indicated by mooring=offside or mooring=towpath.  
mooring_operator and mooring_price tags may be useful. maxstay is a  
good idea.

> Bridges
> -------
> New tag: ref_canal_bridge=<number> for bridge numbers.

Just ref_bridge. Some river navigations have bridge numbers, and I  
wouldn't be too surprised if a few railways did.

> Amenities
> ---------
> New tag value: amenity=sanitary_station

Sanitary station is a really misleading (but sadly widespread) term.  
Better to group all the constituent services  
(amenity=pumpout;water_point), and to come up with a separate tag for  
what we refer to as "Elsan disposal" (a drain where you can empty your  
Porta-Potti!). amenity=poo_hole could be misconstrued.

Stephen Gower wrote:

>   I don't mind there being an assumption that unspecified units are
>   metres, but the UK canals are done in feet, and if I'm going to put
>   any dimensions in, it'll be in feet, so I'd need a way to specify
>   that's what I'd done.

We have a convention that metric units are used unless you explicitly  
specify otherwise... but you _can_ explicitly specify if you want to.  
So:

maxspeed=110   <-- assumed km/h
maxspeed=70mph <-- unit stated
maxwidth=2.14  <-- assumed metres
maxwidht=7ft   <-- unit stated

>   On UK canals, mooring is generally allowed everywhere

(on the towpath side)

>   except where
>   explicity signed otherwise - do we need a tag for
>   mooring-not-allowed?

Good idea.

Final note: suggest we encourage use of the operator tag for the  
navigation authority: operator=British Waterways, operator=VNF, etc.

cheers
Richard





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