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Nominatim/docs/develop/Import.md

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OSM Data Import

OSM data is initially imported using osm2pgsql. Nominatim uses its own data output style 'gazetteer', which differs from the output style created for map rendering.

Database Layout

The gazetteer style produces a single table place with the following rows:

  • osm_type - kind of OSM object (N - node, W - way, R - relation)
  • osm_id - original OSM ID
  • class - key of principal tag defining the object type
  • type - value of principal tag defining the object type
  • name - collection of tags that contain a name or reference
  • admin_level - numerical value of the tagged administrative level
  • address - collection of tags defining the address of an object
  • extratags - collection of additional interesting tags that are not directly relevant for searching
  • geometry - geometry of the object (in WGS84)

A single OSM object may appear multiple times in this table when it is tagged with multiple tags that may constitute a principal tag. Take for example a motorway bridge. In OSM, this would be a way which is tagged with highway=motorway and bridge=yes. This way would appear in the place table once with class of highway and once with a class of bridge. Thus the unique key for place is (osm_type, osm_id, class).