[Tagging] Recognizing backward/forward parts of a road

Paweł Paprota ppawel at fastmail.fm
Mon Aug 6 17:39:12 BST 2012

Hi all,

We (as in Polish OSM community) have been remapping for a while now and
also repairing our road network. As a starting point for this effort we
wanted to create relations for major roads.

It's all good but some "issues" have been coming to light during this
work. One of them I posted last week to this list - data redundancy with
"ref" tag (on relation vs on ways).

Now there is another one which does not seem to be addressed in OSM, at
least not in a consistent manner...

OK, so let's take the following road - it's a Polish motorway (A4):


It is interesting because it has two (one way) parts which don't connect
at any point. Therefore, tools like Relation Analyzer report it is not
in one piece:


Plus it miscalculates length of this road because it counts each part

We have discussed this and decided that it would be good to be able to
easily recognize each "part" of a road in such cases. That's why we
started using backward/forward relation member roles. And our tool
calculates everything properly:


You can see that A4 is green and length is correct (444 km).

However, one person from our community suggested today that We're Doing
It Wrong (tm) because backward/forward roles have different meaning for
route relation members. I am inclined to agree with him 

Basically, backward/forward is used to define in which direction given
way is navigable in given relation - in the context of direction of that
way. So not really a good idea to use those roles for recognizing
different parts of a road.

So the questions are as follows:

1. Is it even needed to be able to recognize (technically) that A4 has
two parts (east-west and west-east - separate)?

2. If "yes" in (1) then how to do it?

Ad. 1 - I think it adds more information and allows to easily calculate
(and more importantly - verify!) length and traverse the road properly.

Ad. 2 - I think relations with proper member roles could work here but
currently it is not specified how to use them in this way.

I have found a previous discussion about exactly this topic:


I guess it is called "super relations" and some people thought it is the
way to go but tools are not handling them well (funnily enough, Relation
Analyzer has been mentioned there too).

Do you think it makes sense to discuss this again? I am willing to do
some work around the topic, without that functionality, reporting on
*roads* (not ways or relations) gets a lot harder.

Please let me know what you think (but constructive stuff only... I'm
not willing to discuss with "OSM is not a computer project" arguments
again... - I am willing to do the computer work so what's wrong with
that if it improves the project?).


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