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).

Configuring the Import

How tags are interpreted and assigned to the different place columns can be configured via the import style configuration file (CONST_Import_style). This is a JSON file which contains a list of rules which are matched against every tag of every object and then assign the tag its specific role.

Configuration Rules

A single rule looks like this:

{
    "keys" : ["key1", "key2", ...],
    "values" : {
        "value1" : "prop",
        "value2" : "prop1,prop2"
    }
}

A rule first defines a list of keys to apply the rule to. This is always a list of strings. The string may have four forms. An empty string matches against any key. A string that ends in an asterisk * is a prefix match and accordingly matches against any key that starts with the given string (minus the *). A suffix match can be defined similarly with a string that starts with a *. Any other string constitutes an exact match.

The second part of the rules defines a list of values and the properties that apply to a successful match. Value strings may be either empty, which means that they match any value, or describe an exact match. Prefix or suffix matching of values is not possible.

For a rule to match, it has to find a valid combination of keys and values. The resulting property is that of the matched values.

The rules in a configuration file are processed sequentially and the first match for each tag wins.

A rule where key and value are the empty string is special. This defines the fallback when none of the rules match. The fallback is always used as a last resort when nothing else matches, no matter where the rule appears in the file. Defining multiple fallback rules is not allowed. What happens in this case, is undefined.

Tag Properties

One or more of the following properties may be given for each tag:

main

A principal tag. A new row will be added for the object with key and value as class and type.
with_name

When the tag is a principal tag (main property set): only really add a new row, if there is any name tag found (a reference tag is not sufficient, see below).
with_name_key

When the tag is a principal tag (main property set): only really add a new row, if there is also a name tag that matches the key of the principal tag. For example, if the main tag is bridge=yes, then it will only be added as an extra row, if there is a tag bridge:name[:XXX] for the same object. If this property is set, all other names that are not domain-specific are ignored.
fallback

When the tag is a principal tag (main property set): only really add a new row, when no other principal tags for this object have been found. Only one fallback tag can win for an object.
operator

When the tag is a principal tag (main property set): also include the operator tag in the list of names. This is a special construct for an out-dated tagging practise in OSM. Fuel stations and chain restaurants in particular used to have the name of the chain tagged as operator. These days the chain can be more commonly found in the brand tag but there is still enough old data around to warrant this special case.
name

Add tag to the list of names.
ref

Add tag to the list of names as a reference. At the moment this only means that the object is not considered to be named for with_name.
address

Add tag to the list of address tags. If the tag starts with addr: or is_in:, then this prefix is cut off before adding it to the list.
postcode

Add the value as a postcode to the address tags. If multiple tags are candidate for postcodes, one wins out and the others are dropped.
country

Add the value as a country code to the address tags. The value must be a two letter country code, otherwise it is ignored. If there are multiple tags that match, then one wins out and the others are dropped.
house

If no principle tags can be found for the object, still add the object with class=place and type=house. Use this for address nodes that have no other function.
interpolation

Add this object as an address interpolation (appears as class=place and type=houses in the database).
extra

Add tag to the list of extra tags.
skip

Skip the tag completely. Useful when a custom default fallback is defined or to define exceptions to rules.

A rule can define as many of these properties for one match as it likes. For example, if the property is "main,extra" then the tag will open a new row but also have the tag appear in the list of extra tags.

There are a number of pre-defined styles in the settings/ directory. It is advisable to start from one of these styles when defining your own.

Changing the Style of Existing Databases

There is normally no issue changing the style of a database that is already imported and now kept up-to-date with change files. Just be aware that any change in the style applies to updates only. If you want to change the data that is already in the database, then a reimport is necessary.