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