Nominatim and the address tags

OpenStreetMap has a well defined schema how to add addresses into the database. There is a whole set of addr:* tags that allow to specify every part of the address in a structured way. When mappers have entered such an address, they naturally expect to be able to search their place later by entering exactly the same terms in the search box. Sometimes they are disappointed.

When it comes to creating addresses, Nominatim has so far mostly ignored the addr:* tags and creates its own address instead. It takes place nodes and administrative boundaries and computes for each place on earth its very unique location. There are a couple of reasons why it is done this way.

First of all, not all objects in OSM have addr:* tags. So we cannot exclusively rely on them to get the address of a place. In a way, the current mechanism is the fallback.

Second, there is the question of translations. Many places in OSM come with translations of their name tags. London for example is translated into 155 languages. Naturally we wouldn’t want to duplicate this onto every single address in London. When Nominatim collects the OSM places to create the address, instead of just looking at the addr:* tags, it can make use of all these translation. This may not sound as important when thinking about your local addresses but it becomes important when looking for places in countries where you cannot read the script. English speakers will surely prefer to see ‘Forbidden City, Xicheng District, Beijing, China’ in the result list instead of ‘Forbidden City, 西城区, 北京市, 中国’.

Finally, there is a technical reason for taking shortcuts with addresses. There are currently about 100 million addresses mapped in OSM. If Nominatim would save the full address for each of them, the size of its database would double. Therefore it cheats a bit and only saves the full address up to street level. When you search for an address like ‘3452 Main St, Crowtown’, Nominatim first looks up ‘Main St, Crowtown’ and then checks that it finds a house number 3452 that belongs to the streets it has found. This saves a lot of space but gets us into trouble with the addr:* tags because those tags belong, of course, to the house number not to the street.

Long story short, your carefully mapped addr:* tags have been ignored by Nominatim all these years. No more.

Last week Nominatim finally has received the ability to search for addr:* tags on addresses and POIs. We still cannot add a separate address entry for each address. Instead the algorithm works a bit differently. Nominatim checks for each address if the names in the addr:* tags are covered already by the street the address belongs to. If that is not the case, then the address gets its very own entry in the search index containing the address elements from the street as well as the content of the addr:* tags. That means that you can now find the address under the name you have put into the tag and under the address that Nominatim has determined on its own.

The change only has an effect on searching. You will find that the display name still contains the address that Nominatim has computed itself. That is because of the translation issue, I mentioned above. There is no good solution for it yet.

As a side-effect of this change, Nominatim also no longer expects that a place mentioned in addr:place exists. The addr:place tag is used for addresses where the house number does not refer to a street. A classic example are small villages that do not have street names but instead just sequentially number their houses. Traditionally Nominatim would assume that addr:place contained a typo and ignore it when there was no OSM place with the same name. Now it just uses the name it finds. An example of such an address is Hainburg, Aussenliegend 13. “Aussenliegend” loosely translates to ‘outside the built-up area of the town’ something that we clearly would not map as a place in OSM.

These changes are already live on nominatim.openstreetmap.org and openstreetmap.org. However, take note that we have not recreated the whole database. So the new entries in the search index only appear, when an address is touched because of your edits to OSM.¹