In the absence of a vast building program of new monorails[1] as proposed by Bill Ricker, I am beginning to think about mapping some of the more complex transport interchanges here in the UK. I am currently adding platforms, walkways and steps to the simpler stations that I know and am now thinking about more complex ones.
Bill also talked about the use of the layer tag on some stations on the monorail at Disney with the stations at layer 3 and the track at layer 2 and I think it would be useful to talk about how we can usefully use layer tags for complex interchanges.
I checked out the IFOPT CEN standard[2] on the subject and found this useful section on "Vertical Levels (Layers) versus Altitude" (they call them levels, we call them layers btw)
1.4.6 Vertical Levels versus AltitudeTransport interchanges are often complex buildings with many interconnected levels. The labelling and description of the levels is used in describing stops and directions in PT info systems and so needs to be part of the Fixed Object model. This LEVEL is a distinct concept from that of a vertical spatial coordinate in that it is a semantic label (for example Departures, Basement , Floor 1, etc). Altitude is in effect the z coordinate of a POINT.
I think this useful for us and clarifies that if the station appears to be organised on two 'layers' (using OSM terminology) then it doesn't matter that in fact one of the layers incorporates an incline at one point so that part of 'layer' it is at a different altitude to another part. If of course if there is a slope which connects two different distinct layers of the station (something which could also occur) then we should consider that to be a path between the layers.
The logic is that when we start mapping more complex 3D stations (and other structures) then we shouldn't get too stressed about altitude, but should instead divide the place up into human understandable layers. This is something that is often done anyway in diagrams that describe the layout of stations[3] and ships[4].
When we come to map this we are going to need an editor that can allow us to see only one level at a time, but there is a growing need for editors to allow one to focus on one aspect of the data only and avoid picking up or modifying features that are out-of-scope anyway so I am sure that will come. While being focused on a particular layer then any features added would be added at that layer.
We would also need to consider slope, steps and lifts between layers and the situation where a lift only connects some layers but not all of them. A lift is currently represented as a single node because it is vertical. How does one indicate which layers it connects? How would one assign layers to a complex metro station where one can't guess the depth of each element - possibly one would have to count steps and measure the height of one of them to calculate a depth for each platform and therefore assign layers.. but that still does solve the problem for escalators.... So, possibly we can be kept occupied even without a massive monorail program.
Fun stuff ;) Peter [1] http://lists.openstreetmap.org/pipermail/talk-transit/2009-September/000593.html [2] http://en.wikipedia.org/wiki/Identification_of_Fixed_Objects_In_Public_Transport [3] http://home.wangjianshuo.com/archives/20080105_shanghai_metro_century_avenue_station.htm [4] http://www.wildalaskacruises.com/capabilities.htm
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