How to find the polygon?

Question

I have a PostGIS database in the "Osm2pgsql/schema".

I get a 7 rows when I query the planet_osm_polygon table with this:

select * from planet_osm_polygon where boundary='administrative' and admin_level = '2' and name = 'Deutschland'

Only 3 of them have a geometry. Unfortunately none of them is the big Germany polygon. They are all enclaves in Belgium or Switzerland. Every single on has an way_area (real type) information. Should I work with this? How?

Or what am I doing wrong? If I do the same for states in Germany, I can't find a single state polygon :(

Accepted Answer

In OSM, polygons, especially large ones, often break because someone makes a small edit somewhere that causes the boundary to be incomplete or self-intersecting, and osm2pgsql may not then be able to build the geometry. It is also possible that the data extract you were using did non contain everything required to build the boundary.

Depending on what you need the polygon for, there are various things you could do.

try a different importer, e.g. imposm
use a different data processing toolchain altogehter, e.g. osmjs which also builds polygons for you
manually retrieve the relation (use a search engine to find "osm relation germany", then issue an API call for /relation/1234/full, then post-process the resulting data - again, depending on what your use case is, perhaps with rel2poly/poly2wkt or something
use a third-party web site that assembles relations for you, e.g. for the area of Germany try ags.misterboo.de where you can download any boundary as GeoJSON.

Having said that, on my osm2pgsql-imported database I get exactly one result for e.g. "admin_level='4' and name='Bayern'", and for "admin_level='2' and boundary='administrative' and name='Deutschland'" I get the following list:

 osm_id | way_area 
--------+----------
 -51477 |  49.2627
 -51477 | 0.001543
 -51477 | 0.000241
 -51477 | 0.000123
 -51477 |    2e-06
 -51477 | 0.001275
 -51477 | 0.000915

Indeed the way_area column can be used as an indicator which of those is the "big Germany polygon".

How to find the polygon?

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