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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 places 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 a secondary importance based on the number of tile views on openstreetmap.org.
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 that conforms to the requirements:
raster2pgsql -I -C -Y -d -t 128x128 input.tiff public.secondary_importance