[OSM-dev] MAPNIK - proposal to drop rendering of underground railway lines

James Mastros james at mastros.biz
Thu May 17 01:52:28 BST 2007

I'm all for displaying all the various forms of public transit on the
map, or at least all the major forms.  One of the primary uses for a
map is to answer the question of "how do I get from A to B", and
in many cities, public transit is a viable way of doing it.

However, doing it well isn't as easy as saying it.  Clearly, there is
something of a value judgement to be made -- we can't start showing
every little bus route everywhere, or the map would be completely
unreadable.  Even if we stick to just major routes, things can get a
bit busy if we render buried (and esp deep buried) lines as full-on
ways, even if at a deep z-level, and pale.

We need an attribute for rail lines, and possibly other things, that
gives the administrative domain of the line.  There's no other
definition that matches usage -- in Munich, and likely much of
Germany, there's no real difference between an S-bahn, a U-bahn, and a
real train then what they decide to call them.  Likewise in London.
Just the same, it's a very important piece of information for a
map-user to know.

Can we even show the lines in the "right" colour without infringing on
LU's (jelously guarded) trademark?  How do we show the difference
between a light blue transit line and a body of water, for example,
when they just happen to be the same color?  How do we specify the
more unusual styles, like dashed lines, and lines with borders?  (They
aren't that rare.)

As a proposal, and I don't know how hard it'd be to implement, we
could show stations with a list of lines they connect to below them,
similar to the way exchanges are shown on the in-train versions of the
LU map.  Even better, we could show short arrows that point toward the
next stations, but that'd likely be quite difficult to implement.

It seems clear to me, though, that transit data is key to any
successful map -- in fact, most data on the map is transit data
already.  Its data where the topology is far, far more important then
the geography, sure, but that doesn't make it any less important to
show, it just means that we need to think a bit harder about how to
show it.

     -=- James Mastros
PS -- I know it's tempting to just think of the London Underground
here, but please, think about all the other ways it's possible to run
a rail service, or even a public transit service.  For example, any
scheme that has a fixed list of types of rail service, especially just
two (national rail and lu) is right out -- German cities commonly have
*at least* three, and four or more if you count trams.

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