[OSM-talk] Proposed automated edit: remove railway=* or highway=* tag on relations tagged type=route

[Guillaume Rischard]

Hello,

Several relations have both type=route and highway=* or railway=* tags; I would like to remove the highway or railway tags from the relations, and leave them on the members. The relation highway tags are redundant with the member ways, make no semantic sense, and can cause rendering issues:

    - Redundancy: A route typically contains highways or railways that each have a highway=* or railway=* tag. Because the information is already included in the members of the relation, it is redundant to tag the relation with that tag as well — you're tagging in two places when you only need one.

    - Semantic meaning: These relations are not highways or railways, they are a *group* of them that make up a route. The tag belongs on the members, not on the relation. It causes semantic problems when the value of the tag differs on the relation and on the member: is the street a motorway like the relation's highway tag says, or residential like the way's tag says?

    - Rendering: if both a way and its relation are tagged as highway=* or railway=*, the line will get drawn twice on the map — once for the way, and once on top of that for the line formed by all the ways in the relation. While this is invisible in most cases, it will cause unexpected results if the tags disagree, e.g. a trunk that's part of a highway=motorway relation, or a railway tunnel that's part of a railway=rail relation.

Since this is a bit abstract, here are concrete examples: http://www.openstreetmap.org/way/25435351 is a tunnel and part of http://www.openstreetmap.org/relation/404214 . Both the way and the relation are tagged as highway=tertiary, but the tunnel isn't shown on the map because of the wrong tag on the relation. http://www.openstreetmap.org/relation/270118 and http://www.openstreetmap.org/way/182573964 show the same issue on railways.

If a country uses highway= [on a way] to reflect a route's administrative classification, then I can see why mappers transfer that to a relation for that route. If, say, the M1 and the M2 both share a stretch of motorway, and you build two relations to express that, it makes sense to tag the M1 relation as highway=motorway. However, the relation *isn't* a highway, but a group of highways.

Taginfo finds type=route combined with 1374 instances of railway=* (0.48%) and 6426 instances of highway=* (2.23%). User pnorman has found relations where the highway relation tag disagrees with one or more members[0] — I have manually fixed some of them while investigating the issue, and there are now 504 left. The same analysis will have to be run for the railway tag, and all of these special cases will have to be looked at and fixed manually.

In the vast majority of these special cases, just deleting the tag or the useless redundant relation will be sufficient, improve the map, and prevent things like service roads on the side of a motorway from being rendered as a motorway. In a minority of cases, the way that disagrees with the relation on highway type shouldn’t be in the relation — the service way that runs parallel to the motorway isn’t part of it.

The edit will be from a specially created imports/mechanical edit account, and follow the Mechanical Edit Policy[1]. Any failed chunks or areas, or all remaining relations the Overpass API finds after running the tool, can simply be re-processed. The different OSM editors should explicitly mark relations tagged as type=route and highway=* or railway=* as an error. If there is no significant negative feedback or issues that cannot be resolved, I will probably open tickets for the various validators and QA tools, and run the fix again every couple of months.

The Overpass API makes it easy to find the relations. The problematic relations exist worldwide, but because they are more common in some regions because of mappers imitating local (bad) habits — France has zero, Bavaria has many, Japan is full of them — I plan on splitting my edits by creating arbitrary groups of 400 relations, or creating tiles and subdividing them up until they contain less than 400 relations. Any better ideas are very welcome.

Guillaume Rischard [2]

[0] http://paste.ubuntu.com/6704102/ using https://gist.github.com/pnorman/cb841edb214a4ab8eb83 on a pgsnapshot database.
[1] http://wiki.openstreetmap.org/wiki/Mechanical_Edit_Policy
[2] htp://openstreetmap.org/user/Stereo/