[OSM-talk] Mapping canals

Gervase Markham gerv-gmane at gerv.net
Thu Jan 17 18:39:19 GMT 2008


As people may know, the UK has an extensive system of canals.
http://www.barging.co.uk/barging_construction_files/CanalMap.gif

There are also dedicated canal map books, which give some idea of how 
canal boaters like canals to be mapped.
http://astore.amazon.co.uk/ukcan0b/203-6807830-9647906?%5Fencoding=UTF8&node=3

Looking at the Map Features section for waterways, it seems that the 
amount of detail available is insufficient. Here follows a sampling of 
issues and queries. Should I make proposals for the following changes?

- waterway=lock_gate might be great for the Manchester Ship Canal, 
doesn't work so well for narrowboat canals. Locks are 70ft long or less, 
and marking the upper and lower gates individually seems unnecessary and 
makes them harder to render. Locks also have names (e.g. "Hatton Bottom 
Lock") which seems to make them better rendered with nodes.

On the other hand, canals have no direction of water flow (with a couple 
of exceptions) but locks are directional. So one might want to indicate 
that by using a short directional way. (But should the way point low to 
high or high to low?) Or perhaps level tags either side of the lock, but 
that could interact badly with bridges if this persisted along the 
canal. (I'd expect the entire canal to have "level=-1" to make creating 
bridges over it easier.) One might also consider "ele" either side, but 
accurate data is hard to obtain with a GPS. Is there best practice in 
this area?

- Locks have a maximum width and length, universally measured in feet. 
It should be possible to indicate what it is, presumably using maxlength 
and maxwidth. Is there a danger of these being rendered with symbols 
appropriate only for roads?

- Sometimes you have a "staircase lock", where the exit of one is the 
entrance of another. These need denoting.

- It is useful to denote the rise/fall of a lock (again, universally 
measured in feet). There needs to be a tag for this. We could 
appropriate "depth", but it's not actually the depth of the canal.

- waterway=aqueduct should surely be applicable to ways rather than nodes?

- waterway=mooring should also be applicable to ways, so that the length 
of the mooring can be seen. It should be possible to indicate what side 
of the canal the mooring is on (perhaps by reference to the towpath 
side?), and any conditions attached to mooring there (usually a maxstay, 
measured in days, or perhaps a fee).

- Navigation on canals is done by means of bridge numbers. All bridges, 
including those between two fields (and so with no associated road) have 
a number. There needs to be a tag to associate a bridge number with the 
short "bridge=yes" way.

- Sometimes bridges which no longer exist (but you can see the remains) 
have numbers. How could this be denoted?

- Some bridges are swing bridges, which means they need to be moved out 
of the way, by one of various means; these need notating. "bridge_type"?

- All canals have towpaths. These paths are of varying quality, and this 
information is useful to walkers and cyclists. The towpath can be on 
either side. How should the towpath be denoted? As a separate way 
parallel to the canal? What tags should be used? 
("highway=footway,bicycle=yes")? How should quality be denoted?

- There are several sorts of "waste disposal" on canals - rubbish 
points, where bags of refuse can be left, and "sanitary stations" and 
"pumpout" points, where different types of toilet can be emptied. 
waterway=waste_disposal will not do for all three. (In fact, the 
description - "A place to release used water e.g. for caravans" - fits 
neither.) New tags are needed.

- Canals sometimes have narrow sections, perhaps where a bridge used to 
be. These are useful to know about and for navigation. Are these best 
denoted with maxwidth? All you really need is an on/off toggle for a 
section.

- Tunnels often have rules about who can enter and when (e.g. transit 
north beginning between :00 and :15; south between 30: and :45). We have 
"hour_on" and "hour_off" - is that enough? How should such tags be 
applied, given that the restrictions are different in each direction?

- Turning points ("winding holes") have a maximum length; this could be 
done using "maxlength" on the node, but that's confusing, because if the 
node is part of the canal, it implies that only boats below that length 
can pass that part of the canal. Which is wrong. So how should this be 
handled?

- There are "mile markers" along the canal which are useful for 
navigation and as points of reference and interest.

And this is just a start :-)

Gerv





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