How can you get country boundaries with flex output?

[iboates]

I am really struggling to get country boundaries when using the flex output.

I filtered my pbf with osmium:

osmium tags-filter final.pbf n/amenity=charging_station admin_level "-o", filtered_pbf,

This should give me all objects with any value at all in admin_level (in addition to the charging stations

And then I try to turn them into polygons in the flex style file:

function osm2pgsql.process_way(object)

tables.country:insert({
    name = object.tags["name:en"],
    geom = object.as_polygon()

})

But i only get some selected polygons as a result. This table is huge, with many tens of thousands of entries, but only a few have geometry:

I don't see any examples of this in the example lua files, is there some trick to this I'm not getting?

[lonvia]

Boundaries are mapped as relations in OSM (and only relations), so you need to use the process_relation() function. Also have a look at the not_null flag for geometry columns.

And finally, for country boundaries better filter by boundary=administrative. People have funny ideas, where they add an admin_leveltag. Give the --remove-tags switch a try. It is another great way to reduce the amount of noise from referenced objects (e.g. ways in your boundary relation which you only want for the geometry).

[iboates]

Thanks. I was able to get part of the way there, but I can't quite figure out how I can iterate over the members of the relation to check if they are ways, and then insert them (or potentially chain them together so I can polygonize them later).

In the end I just inserted them as geometry collections and then wrote an SQL query to process the result into polygons. It's clunky but it works.

I was wondering if you have any tips on how I can better traverse the members of the relations. Because this was as far as I could make it on my own:

(PBF pre-filtered with "n/amenity=charging_station" and "boundary=administrative" in osmium)

flex script:

local srid = 4326

local tables = {}

tables.country_pre = osm2pgsql.define_way_table('country_pre', {
    { column = 'name', type = 'text' },
    { column = 'tags', type = 'jsonb' },
    { column = 'geom', type = 'geometrycollection', projection = srid }
})

function osm2pgsql.process_relation(object)

    if object:grab_tag('admin_level') == "2" then
    tables.country_pre:insert({
        name = object.tags["name:en"],
        tags = object.tags,
        geom = object:as_geometrycollection()
    })
    end

end

Postprocessing query:

drop table if exists country;
create table country as (

    with a as (
        select
            st_asewkt((st_dump(geom)).geom) as geom,
            st_geometrytype((st_dump(geom)).geom) as geom_type,
            name
        from
            country_pre
    ),

    b as (
        select
            ST_Node(st_collect(geom)) as geom,
            name
        from
            a
        where
            geom_type = 'ST_LineString'
        group by
            name
    )

    select
        ST_SimplifyPreserveTopology((ST_Dump(st_polygonize(geom))).geom, 0.00005) as geom,
        name
    from
        b
    group by
        name
);

drop table country_pre;

create index on country (geom) using gist;

[joto]

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 gravitystorm/openstreetmap-carto#4431 .