[Tagging] direction=forward/backward on nodes ?

Peter Wendorff wendorff at uni-paderborn.de
Mon Apr 14 06:28:01 UTC 2014


Am 13.04.2014 21:35, schrieb Steve Doerr:
> I'm surprised that so many people are jumping to this conclusion. Let's
> remember that a way is just a series of nodes in a particular order. So
> a node is not necessarily an isolated object. 
> In many cases, it exists solely as part of a way. Thus the concept of direction is not
> meaningless for a node which is part of a way. 
Agree partly. It's not meaningless, but it get's ambiguous very often.
Take traffic signals as one example where the direction might be used:
Besides an intersection someone could add the traffic lights on the four
individual ways (instead on the intersecting node itself).
This matches the installation of the individual lights and the stop
positions, but it produces wrong results without a direction tag.

The drawback of that is, that someone crossing the intersection straight
meets two traffic lights, which is wrong of course. The mapper therefore
might decide to add direction-tags to them, as each traffic light node
is relevant and applied only for one of the two directions.

Looks perfect now - all four traffic lights are mapped separately where
they are, routing for cars works great (provided that the direction tag
is known and supported by routers).

Enter of the next mapper: He want's to add the footways and cycleways
that cross the streets using the pedestrian traffic lights integrated in
the traffic lights mentioned above.
As a result the nodes previously mapped with a direction are shared by
two ways, and it's hard to determine what the direction tag refers to,
as of course crossing for pedestrians is possible and meaningful for
both directions.

> I haven't examined any
> uses of the tag on a node, but I can imagine, for instance, that a node
> in a way with a direction attribute might be used to represent a
> road-sign that applies only to traffic on the way passing that node in a
> particular direction.
For other traffic signs it's the same, and that's why we usually map the
road signs meaning on the road that is affected by it. (The sign itself
may be mapped as such, as an obstacle and a physical object next to the
street), while maximum speed, maximum dimensions (width, height,
weight), oneway access, access restrictions and so on are mapped on the
where they hold.

Here the direction is useful (look at the oneway=yes tag), meaningful
and not ambiguous; on nodes it is or get's very lightly without tagging


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