[Tile-serving] [openstreetmap/osm2pgsql] How can you get country boundaries with flex output? (Discussion #1934)

[Jochen Topf]

The simplest way to handle boundaries is to create multipolygons from the boundary relations in osm2pgsql. Then you can either just draw the outline of those multipolygons or turn them into (multi)linestrings in the database. You can also create multilinestrings directly from the relations if your osm2pgsql version is new enough.

This is often good enough if you only want to render one level of boundaries (i.e. only country boundaries). But if you want to render multiple levels (country, state, ...) or want to render the boundaries half-transparently, you'll get intro trouble. Then you really need to handle each piece of boundary separately. This is much more effort and you'll need some SQL magic for it and/or use 2-stage processing in osm2pgsql Lua which is a bit painful. I don't have a good out-of-the-box solution there, although I believe @pnorman has support for this in https://github.com/gravitystorm/openstreetmap-carto/pull/4431 .

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