[Talk-transit] Vertical Levels (Layers) versus Altitude for stations

[Peter Miller]

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 Altitude
Transport 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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