Country boundaries polygons for country-appending points. Based on Movisda's coastal-buffered polygons.

create-country-boundaries-parquet.sh

#!/usr/bin/env bash

set -euo pipefail

# Movisda is a great source for admin boundaries because they have relatively
# precise land boundary tracing, and buffered coastal boundary tracing, so we
# don't miss coastal points that would fall outside imprecise boundaries.
curl -O https://osm.download.movisda.io/admin/Admin-latest.geojson

# This query selects country polygons (admin_level = 2) and resolves overlaps.
# For any two intersecting polygons, the overlapping area is subtracted
# from the polygon that has the higher `osm_id`, ensuring no gaps are created.
# ST_Overlaps is used instead of ST_Intersects to only consider significant
# area-based overlaps, not simple boundary touches.
# ST_MakeValid is used to fix any invalid geometries in the
# source data, which resolves TopologyExceptions.

datestamp=$(date +%Y%m%d)
query="
	load spatial;
	copy (

		with countries as (
			select
				*,
				st_makevalid(geom) as valid_geom
			from st_read('Admin-latest.geojson')
			where admin_level = 2
		),
		areas_to_subtract as (
			select
				c1.osm_id,
				st_union_agg(c2.valid_geom) as clipping_geom
			from
				countries as c1
			join
				countries as c2 on st_overlaps(c1.valid_geom, c2.valid_geom) and c1.osm_id > c2.osm_id
			group by
				c1.osm_id
		)
		select
			c.osm_id,
			c.osm_version,
			c.prefix,
			c.bytes,
			c.subareas,
			c.timestamp,
			c.admin_level,
			c.name_en,
			c.name,
			c.cx,
			c.cy,
			coalesce(st_difference(c.valid_geom, ats.clipping_geom), c.valid_geom) as geom
		from
			countries as c
		left join
			areas_to_subtract as ats on c.osm_id = ats.osm_id

	) to 'boundaries-${datestamp}.parquet'
	;
"

duckdb -c "$query"