Nominatim/Development overview

For more up-to-date information about the internals of Nominatim, check the Developer's documentation.

Overview

Nominatim comprises of:

• osm2pgsql output routine
• Postgresql module
• Set of plpgsql functions
• PHP management / interface

Nominatim indexes all named features and a selection of points of interest and provides search, reverse geocoding and some  gazetteer functionality.

Tags processed

For a list of the tags processed by Nominatim, check the import style file.

Indexing/address calculation

Country to street level

All indexed features are converted to a simple hierarchy (rank) of importance with points scored between 0 and 30 (where 0 is most important). Rank takes account of differences in interpretation between different countries but is generally calculated as:

For administrative boundaries: admin_level * 2	4–22
Continent, sea	2
Country	4
State	8
Region	10
County	12
City	16
Island, town, moor, waterways	17
Village, hamlet, municipality, district, borough, airport, national park	18
Suburb, croft, subdivision, farm, locality, islet	20
Hall of residence, neighbourhood, housing estate, landuse (polygon only)	22
Airport, street, road	26
Paths, cycleways, service roads, etc.	27
House, building	28
Postcode	11–25 (depends on country)
Other	30

For each feature down to level 26 (street level) a list of parents is calculated using the following algorithm:

1. All polygon/multi-polygon areas which contain this feature (in order of size).
2. All items by name listed in the is_in are searched for within the current country (in no particular order).
3. The nearest feature for each higher rank, and all others within 1.5 times the distance to the nearest (in order of distance).

and a list of keywords is generated from those features.

During the indexing process, an address is also calculated using the first feature found for each level. Where an is_in value is provided it is used to filter the address.

Building indexing

Buildings, houses and other lower than street level features (i.e., bus stops, phone boxes, etc.) are indexed by relating them to their most appropriate nearby street.

The street is calculated as:

1. The street member of an associatedStreet relation
2. If the node is part of a way:
   1. If this way is street level than that street
   2. The street member of an associatedStreet relation that this way is in
   3. A street way with 50/100 meters and parallel with the way we are in
3. A nearby street with the name given in addr:street of the feature we are in or the feature we are part of
4. The nearest street (up to 3 miles)
5. Not linked

All address information is then obtained from the street. As a result addr:* tags on low-level features are not processed (except as above).

For interpolated ways simple numerical sequences are extrapolated (alphanumerical sequences are not currently handled) and additional building nodes are inserted into the way by duplicating the first (lowest) house number in the sequence.