[Tagging] The direction=* tag

[Daniel Hofmann]

Jumping in here to give a perspective from a routing engine (OSRM,
https://github.com/Project-OSRM/osrm-backend#open-source-routing-machine).
We do not handle direction tags on nodes which indicate a property for a
way or a turn at an intersection. The example with stop signs and give
yield signs is spot on. Even worse is the assumption that routing engines
can just infer the direction by checking the distance to the nearest
intersection. This is in conflict with how parsing and creating a graph
works.

There is a similar problem with exit_to node tags, indicating the exit way
destination - you can read about it here

- https://www.openstreetmap.org/user/daniel-j-h/diary/40555 (en)
- https://www.openstreetmap.org/user/daniel-j-h/diary/40554 (de)

Cheers,
Daniel J H


On Fri, Mar 17, 2017 at 11:25 AM, Martin Koppenhoefer <
dieterdreist at gmail.com> wrote:

>
> 2017-03-16 5:13 GMT+01:00 Tod Fitch <tod at fitchdesign.com>:
>
>> The “direction” tag [1] has different uses that seem disjoint to me.
>>
>>    1. To specify the orientation (compass point or degrees from north)
>>    of an object (adit or cave entrance, etc.).
>>
>>
> "orientation" would have been a better descriptor IMHO, but the crowd uses
> this tag differently (see taginfo, also subtags like roof:orientation,
> ...). Direction is working for me nonetheless.
>
>
> 2. To specify direction (clockwise/counterclockwise) around a roundabout
>> (not sure why this is needed as it should be apparent from local laws or
>> specified with a “oneway=yes”).
>
>
>
> agree with you
>
>
> 3. To indicate the direction (forward/backward) a stop or yield (give way)
>> sign has effect along a way.
>
>
>
> broken. From time to time people are coming up with features to tag on
> nodes that require (or seem to require) the information of a direction.
> Taking the direction of a different object (e.g. here a way) doesn't seem a
> healthy way to represent this. Ways might get split, might get reversed,
> nodes might be (or become) part of several ways, etc. Either use a cardinal
> direction or a short way stub or a relation, etc., but not "forward" or
> "backward" tag values on a node, it simply doesn't make sense. Tags should
> refer to the object they are tagged on.
>
>
>
>
>> Oddly, that third use seems only for stop and yield signs but not for
>> traffic signals where a “traffic_signals:direction=forward | backward”
>> tag is to be used. However that seems to be the most used form [2].
>> Apparently some have figured that if we have “traffic_signals:direction”
>> there should be “stop:direction” [3] and “give_way:direction” [4] tags.
>>
>
>
> similarly broken
>
>
> I would keep the variant 1 and discourage 2 and 3.
>
> Cheers,
> Martin
>
>
>
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