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78 lines
3.0 KiB
Markdown
78 lines
3.0 KiB
Markdown
# Fallback Country Boundaries
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Each place is assigned a `country_code` and partition. Partitions derive from `country_code`.
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Nominatim imports two pre-generated files
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* `data/country_name.sql` (country code, name, default language, partition)
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* `data/country_osm_grid.sql` (country code, geometry)
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before creating places in the database. This helps with fast lookups and missing data (e.g. if the data the user wants to import doesn't contain any country places).
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The number of countries in the world can change (South Sudan created 2011, Germany reunification), so can their boundaries. This document explain how the pre-generated files can be updated.
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## Country code
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Each place is assigned a two letter country_code based on its location, e.g. `gb` for Great Britain. Or `NULL` if no suitable country is found (usually it's in open water then).
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In `sql/functions.sql: get_country_code(geometry)` the place's center is checked against
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1. country places already imported from the user's data file. Places are imported by rank low-to-high. Lowest rank 2 is countries so most places should be matched. Still the data file might be incomplete.
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2. if unmatched: OSM grid boundaries
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3. if still unmatched: OSM grid boundaries, but allow a small distance
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## Partitions
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Each place is assigned partition, which is a number 0..250. 0 is fallback/other.
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During place indexing (`sql/functions.sql: placex_insert()`) a place is assigned the partition based on its country code (`sql/functions.sql: get_partition(country_code)`). It checks in the `country_name` table.
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Most countries have their own partition, some share a partition. Thus partition counts vary greatly.
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Several database tables are split by partition to allow queries to run against less indices and improve caching.
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* `location_area_large_<partition>`
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* `search_name_<partition>`
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* `location_road_<partition>`
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## Data files
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### data/country_name.sql
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Export from existing database table plus manual changes. `country_default_language_code` most taken from [https://wiki.openstreetmap.org/wiki/Nominatim/Country_Codes](), see `utils/country_languages.php`.
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### data/country_osm_grid.sql
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`country_grid.sql` merges territories by country. Then uses `function.sql: quad_split_geometry` to split each country into multiple [Quadtree](https://en.wikipedia.org/wiki/Quadtree) polygons for faster point-in-polygon lookups.
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To visualize one country as geojson feature collection, e.g. for loading into [geojson.io](http://geojson.io/):
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```
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-- http://www.postgresonline.com/journal/archives/267-Creating-GeoJSON-Feature-Collections-with-JSON-and-PostGIS-functions.html
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SELECT row_to_json(fc)
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FROM (
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SELECT 'FeatureCollection' As type, array_to_json(array_agg(f)) As features
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FROM (
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SELECT 'Feature' As type,
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ST_AsGeoJSON(lg.geometry)::json As geometry,
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row_to_json((country_code, area)) As properties
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FROM country_osm_grid As lg where country_code='mx'
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) As f
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) As fc;
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```
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`cat /tmp/query.sql | psql -At nominatim > /tmp/mexico.quad.geojson`
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