[OSM-newbies] Administrative boundaries

Thomas Wood grand.edgemaster at gmail.com
Sun Nov 9 11:43:17 GMT 2008


2008/11/9 James Ewen <jewen at shaw.ca>:
> On Fri, Nov 7, 2008 at 6:56 PM, Simone Cortesi <simone at cortesi.com> wrote:
>
>> there is a good consensus on the fact that using a relation is a good
>> thing for borders:
>> http://wiki.openstreetmap.org/index.php/Relations/Proposed/Boundaries
>
> That leaves a lot to be desired for a description... After many hours
> of contemplation, and looking at the way surrounding the Vatican, I
> think I understand what it is that is being described.
>
> Rather than create yet another way to describe a border, one would
> rather simply include all the existing ways in a relation. The
> relation defining a border may include other ways that define roads,
> edges of woodland, rivers, or ways that only define the actual border.

No, the renderers currently only look for actual boundary ways. The
purpose of the relation is to tie boundary lines together so polygons
can be formed if required.

>> or take a look at what I used when importing italian borders few weeks
>> ago: http://wiki.openstreetmap.org/index.php/Italian_Borders
>>
>> you need for the way:
>> <tag k="boundary" v="administrative" />
>> <tag k="admin_level" v="a_number_2,4,6 or 8" />
>>
>> for the relation:
>> <tag k="type" v="boundary" />
>> <tag k="boundary" v="administrative" />
>
> The boundary page describes this format:
>
> boundary=administrative
> + admin_level=2
>
> But how do you apply either of these in the real world? My tool of
> choice is Potlatch.
>
> So, if I took 4 ways (roads for example) that defined a quadrilateral,
> and added them to a relation, and then defined that relation as a
> boundary, with an appropriate admin_level, I should get a border drawn
> on the map.

This wouldn't work (although very nice conceptually), as noted, the
renderers only look for boundary=administrative ways tagged as such at
the moment.

The original plan for boundaries is that you draw a way representing
the whole boundary of a region. However, this was unfeasible for
mapping sections of a boundary from ground sources, as opposed to mass
imports. It also had the consequence of very large ways being produced
that data clients were unable to handle.

The alternative method was proposed of marking a boundary way between
two regions. When the region on one side of the boundary changes, you
should start a new way (since at this point there'd be an intersection
of at least 3 boundaries). You'd define what was on either side of the
boundary by using region_name:left and region_name:right tags

Relations were then added to the mix - the boundary relation was
proposed to tie up the boundary ways of an area together and use that
to name the area they represent.

Currently a mix of methods are being used. Generally all require ways
with boundary=administrative and admin_level=<number> to be rendered.

And finally, a more exact link of where you're editing will be of help
- both Strathcona County and the bits of the Alberta border you've
tried defining are quite difficult to find if you've got little
knowledge of Canada!

Regards,
Thomas Wood




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