X-Git-Url: https://git.openstreetmap.org./nominatim.git/blobdiff_plain/96a84294f435c99dc67021dccb11b0b07d6b0508..90a5d2301610142994e75311be8ef9ca4643aafb:/docs/develop/Ranking.md

diff --git a/docs/develop/Ranking.md b/docs/develop/Ranking.md
index 03704937..5b1ca191 100644
--- a/docs/develop/Ranking.md
+++ b/docs/develop/Ranking.md
@@ -7,24 +7,74 @@ different purposes, which are explained in this chapter.
 ## Search rank
 
 The search rank describes the extent and importance of a place. It is used
-when ranking search result. Simply put, if there are two results for a
+when ranking search results. Simply put, if there are two results for a
 search query which are otherwise equal, then the result with the _lower_
 search rank will be appear higher in the result list.
 
 Search ranks are not so important these days because many well-known
 places use the Wikipedia importance ranking instead.
 
+The following table gives an overview of the kind of features that Nominatim
+expects for each rank:
+
+rank   | typical place types             | extent
+-------|---------------------------------|-------
+1-3    | oceans, continents              | -
+4      | countries                       | -
+5-9    | states, regions, provinces      | -
+10-12  | counties                        | -
+13-16  | cities, municipalities, islands | 15 km
+17-18  | towns, boroughs                 | 4 km
+19     | villages, suburbs               | 2 km
+20     | hamlets, farms, neighbourhoods  |  1 km
+21-25  | isolated dwellings, city blocks | 500 m
+
+The extent column describes how far a feature is assumed to reach when it
+is mapped only as a point. Larger features like countries and states are usually
+available with their exact area in the OpenStreetMap data. That is why no extent
+is given.
+
 ## Address rank
 
 The address rank describes where a place shows up in an address hierarchy.
 Usually only administrative boundaries and place nodes and areas are
-eligible to be part of an address. All other objects have an address rank
-of 0.
-
-Note that the search rank of a place place a role in the address computation
-as well. When collecting the places that should make up the address parts
-then only places are taken into account that have a lower address rank than
-the search rank of the base object.
+eligible to be part of an address. Places that should not appear in the
+address must have an address rank of 0.
+
+The following table gives an overview how ranks are mapped to address parts:
+
+ rank        | address part
+-------------|-------------
+ 1-3         | _unused_
+ 4           | country
+ 5-9         | state
+ 10-12       | county
+ 13-16       | city
+ 17-21       | suburb
+ 22-24       | neighbourhood
+ 25          | squares, farms, localities
+ 26-27       | street
+ 28-30       | POI/house number
+
+The country rank 4 usually doesn't show up in the address parts of an object.
+The country is determined indirectly from the country code.
+
+Ranks 5-24 can be assigned more or less freely. They make up the major part
+of the address.
+
+Rank 25 is also an addressing rank but it is special because while it can be
+the parent to a POI with an addr:place of the same name, it cannot be a parent
+to streets. Use it for place features that are technically on the same level
+as a street (e.g. squares, city blocks) or for places that should not normally
+appear in an address unless explicitly tagged so (e.g place=locality which
+should be uninhabited and as such not addressable).
+
+The street ranks 26 and 27 are handled slightly differently. Only one object
+from these ranks shows up in an address.
+
+For POI level objects like shops, buildings or house numbers always use rank 30.
+Ranks 28 is reserved for house number interpolations. 29 is for internal use
+only.
 
 ## Rank configuration
 
@@ -37,9 +87,9 @@ into the database. There are a few hard-coded rules for the assignment:
     * highway nodes
     * landuse that is not an area
 
-Other than that, the ranks can be freely assigned via the json file
-defined with `CONST_Address_Level_Config` according to their type and
-the country they are in.
+Other than that, the ranks can be freely assigned via the JSON file according
+to their type and the country they are in. The name of the config file to be
+used can be changed with the setting `NOMINATIM_ADDRESS_LEVEL_CONFIG`.
 
 The address level configuration must consist of an array of configuration
 entries, each containing a tag definition and an optional country array:
@@ -78,12 +128,13 @@ definition is used as a fallback, when nothing more specific for a given
 country exists.
 
 `tags` contains the ranks for key/value pairs. The ranks can be either a
-single number, in which case they are to search and address rank, or a tuple
+single number, in which case they are the search and address rank, or an array
 of search and address rank (in that order). The value may be left empty.
 Then the rank is used when no more specific value is found for the given
 key.
 
-Countries and key/value combination may appear in multiple defintions. Just
-make sure that each combination of counrty/key/value appears only once per
+Countries and key/value combination may appear in multiple definitions. Just
+make sure that each combination of country/key/value appears only once per
 file. Otherwise the import will fail with a UNIQUE INDEX constraint violation
 on import.
+