[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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