[OSM-dev] Extracting national boundaries

[Colin Smale]

  Hi,

I am trying to extract national boundaries from OSM and it's proving 
more difficult than I anticipated.

What I would like to end up with, is a collection of complex polygons 
(inner and outer rings including enclaves/exclaves), one for each 
country. I thought I should be able to do that by getting all relations 
with type=boundary, admin_level=2. Using XAPI I got a large file which 
contains some of the information, but as some national boundaries seem 
to be implemented using subrelations and subsubrelations etc I now find 
I will have to query the database for each subrelation individually and 
drill down successively until I have all the ways I need. But that looks 
as if it will give me large numbers of internal borders between 
lower-level admin units as well, and it will be an enormous job to 
stitch everything together and discard the internal borders.

For many countries there is a simple relation for the national boundary, 
referencing a number of ways, which I can concatenate to produce the 
complete outer boundary. But France and Slovakia for example don't seem 
to have a single relation as a starting point.

I can't imagine I am the first person to want to do this. Has anyone got 
any tips? Or maybe even a ready-made data set?

National boundaries don't change all that often, but it does happen from 
time to time, so it would be nice if this could be repeatable in the future.

Thanks in advance for any help you can give!

Regards,
Colin