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Importance
Search requests can yield multiple results which match equally well with the original query. In such case Nominatim needs to order the results according to a different criterion: importance. This is a measure for how likely it is that a user will search for a given place. This section explains the sources Nominatim uses for computing importance of a place and how to customize them.
How importance is computed
The main value for importance is derived from page ranking values for Wikipedia pages for a place. For places that do not have their own Wikipedia page, a formula is used that derives a static importance from the place's search rank.
In a second step, a secondary importance value is added which is meant to represent how well-known the general area is where the place is located. It functions as a tie-breaker between places with very similar primary importance values.
nominatim.org has preprocessed importance tables for the primary Wikipedia rankings and for secondary importance based on Wikipedia importance of the administrative areas.
The source code for creating these files is avaible in the Github projects osm-search/wikipedia-wikidata and osm-search/secondary-importance.
Customizing secondary importance
The secondary importance is implemented as a simple
Postgis raster table, where Nominatim
looks up the value for the coordinates of the centroid of a place. You can
provide your own secondary importance raster in form of an SQL file named
secondary_importance.sql.gz in your project directory.
The SQL file needs to drop and (re)create a table secondary_importance which
must as a minimum contain a column rast of type raster. The raster must
be in EPSG:4326 and contain 16bit unsigned ints
(`raster_constraint_pixel_types(rast) = '{16BUI}'). Any other columns in the
table will be ignored. You must furthermore create an index as follows:
CREATE INDEX ON secondary_importance USING gist(ST_ConvexHull(gist))
The following raster2pgsql command will create a table from a tiff file that conforms to the requirements:
raster2pgsql -I -C -Y -d -t 128x128 input.tiff public.secondary_importance