[Talk-us] Separate relations for each direction of US & State highways.

[Chris Lawrence]

On Wed, Nov 20, 2013 at 2:32 PM, Martijn van Exel <martijnv at telenav.com>wrote:

> Reading through this I see that most are in favor of avoiding dividing
> relations more than necessary: no separate relations for directions,
> especially not if the geometry is the same for both.
>
> That leaves the question of how to tag cardinal directions? As I said
> before, I think the role tag is suitable for this. It's already used
> pretty widely (with around 100k ways having a cardinal direction as
> the role tag in a relation), documented
> (https://wiki.openstreetmap.org/wiki/Route#Members) and it replaces
> the forward / backward role values that are not particularly useful or
> meaningful.
> I created a stub of a wiki page that we could use to describe the
> preferred tagging in more detail:
> http://wiki.openstreetmap.org/wiki/Highway_Directions_In_The_United_States
> .
> (The current content is geared towards using the member role tag, but
> this can be changed depending on the outcome of this discussion.)
>

The only problem I can anticipate with this tagging scheme is that it's
possible some editors don't understand anything other than
left/right/forward/backward (I think), we could end up in data loss
situations fairly easily.

For example: way X pointing east is marked in relation Y as "east"
(presumably we could assume that "east" = forward and the opposite cardinal
direction "west" is backward). User reverses way X. Now the relation role
is potentially backward.  JOSM seems to understand at least north/south and
east/west and offers to fix it (see
http://josm.openstreetmap.de/browser/josm/trunk/src/org/openstreetmap/josm/corrector/ReverseWayTagCorrector.java);
no idea if iD or Potlatch do.

We'd also need to make the validation tools smarter to recognize lossage
(for example, realizing that the route is unbroken only if the chain of
role tags once you account for the directions of the underlying ways is
monotonic),


Chris
-- 
Chris Lawrence <lordsutch at gmail.com>

Website: http://www.cnlawrence.com/
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