[OSM-talk-be] some thoughts and remarks about border mapping

[Lennard]

Ben Laenen wrote:

> The current system doesn't allow for much nowadays. If something is the 
> boundary between provinces, and it's also the boundary between 
> municipalities, it should be tagged with the admin_level of the 
> province. That's the current idea, and it makes province borders bigger 
> than municipality borders on the maps.

It also makes for holes in the municipal border if you extract only that 
admin_level from the db. For instance to make a map of only the 
municipal borders in Belgium.

> So that gives you a raster of boundaries. My idea is that these should 
> be combined together into relations that have the admin_level of the 
> thing its ways enclose. That way you can see that boundary X is a 
> province, arrondissement and municipality boundary for example. This 
> has never been agreed on, but I think it's somehow applied on the Dutch 
> border.

The Dutch border system as it is now is comprised of overlapping ways 
using shared nodes. The municipal border is a closed way, the province 
border is another closed way, and it shares the nodes with various 
municipal boundaries. And on a higher level, the national border is 
another closed way that shares the nodes with the respective provinces 
and municipalities along the border.

Note that this system isn't ideal either, since if you work on a small 
bbox (area) near a boundary, the API will give you the entire boundary, 
reaching far outside the bbox. Ideally, these long borders should be 
split up, and put into relations. In this relation system, you don't 
need the overlapping ways either. If you have a municipal border that's 
also the province and/or even the national border, you only need -1- 
way, which will be a member of 2 or 3 relations.

Or even a member of 6 relations: border of the resp. BE and NL 
municipalities (2x), border of the resp. BE and NL provinces (another 
2x), and national border of both BE and NL (for the final 2x relations). 
A total of 6 relations.

Substitute NL for FR/DE/LU if you live near there. :)

> In a truly happy world the boundary ways don't need admin_level and 
> should be extracted from its relations.

Exactly, but is that already supported by the renderers at the moment? 
I'd be very interested to know this, because I'm also in the process of 
adding bits of municipal borders here and there, and would like to use 
relations.

> Of course, all these relations have its drawbacks: Potlatch especially 
> becomes awfully slow if there are a lot of relations in view

Don't let an editor stand in the way of smart tagging.

-- 
Lennard