[OSM-dev] Getting admin_level=* from relations to render properly

[Lennard]

On 28-4-2010 8:28, John Smith wrote:

> It occurred to me a few weeks ago that ways shouldn't be tagged with
> the admin_level, but instead use the information from relations, but

How did this occur to you? Tagging the ways is even explicitly 
documented on the wiki.

> this doesn't work, in fact I removed the admin_level tag from a way
> that makes up part of a state border assuming the information would be
> used from the relation instead, but it just didn't render at all until
> I reinstate the admin_level on the way.

If the relation forms a closed loop, and is tagged as a boundary, it 
should currently render.

> I haven't figured out what needs to be fixed in the mapnik style sheet
> to fix this, can anyone tell me what needs tweaking to fix this?

Should work currently. The major drawback to also drawing boundary 
relations is that they can stack (in osm2pgsql+mapnik). Where a tagged 
boundary way is part of n boundary relations, you will see n+1 
overlapping lines in the render. IMO this is ugly, and you get no sense 
of the actual admin levels involved.

Rendering just the boundary way makes for a clean border render, with a 
predictable appearance. To this end, the wiki documentation describes 
tagging the way with the highest order (lowest value) of admin_level. As 
far as I'm aware, Tiles at Home already take this approach, and does not 
render boundary relations.

I trust people will now bring the "don't tag for the renderer" mantra 
into play, but in my mind, a boundary relation(*) describes the polygon, 
so the administrative *area*, and the ways describe the demarcation 
between administrative areas. Thus, it's the ways that should appear on 
the rendered map, not the area.

*) Actually tagged as a multipolygon, because, hey, that's what it 
really is.

-- 
Lennard