[Talk-us] understanding administrative boundary relations

[Toby Murray]

On Fri, Sep 4, 2015 at 1:37 PM, Ray Kiddy <ray at ganymede.org> wrote:
>
> Strangely, I am finding that some of the cities in California _are_ in
> the system, but as ways and not as relations. This seems odd, but we
> will see.

Well the primary reason to use relations in boundaries is to reduce
duplication. So if two cities share a border, the same way can be used
in both relations. Sometimes people even use roads or streams or other
physical ways as part of boundary relations. I personally usually
avoid this because I like having boundary relations completely
separate from other things so that they are easier to update in the
future. So for a city that is not part of a metro area with adjoining
cities, it is perfectly fine to just used a closed way instead of a
relation for the boundary. At the end of the day, both ways and
relations generally get turned into either linestrings (if linear) or
multipolygons (if closed) in things like a postgis database or a
shapefile.

Toby