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docs/develop/Ranking.md

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+# Place Ranking in Nominatim
+
+Nominatim uses two metrics to rank a place: search rank and address rank.
+Both can be assigned a value between 0 and 30. They serve slightly
+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
+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.
+
+## 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.
+
+## Rank configuration
+
+Search and address ranks are assigned to a place when it is first imported
+into the database. There are a few hard-coded rules for the assignment:
+
+ * postcodes follow special rules according to their length
+ * boundaries that are not areas and railway=rail are dropped completely
+ * the following are always search rank 30 and address rank 0:
+    * 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.
+
+The address level configuration must consist of an array of configuration
+entries, each containing a tag definition and an optional country array:
+
+```
+[ {
+    "tags" : {
+      "place" : {
+        "county" : 12,
+        "city" : 16,
+      },
+      "landuse" : {
+        "residential" : 22,
+        "" : 30
+      }
+    }
+  },
+  {
+    "countries" : [ "ca", "us" ],
+    "tags" : {
+      "boundary" : {
+        "administrative8" : 18,
+        "administrative9" : 20
+      },
+      "landuse" : {
+        "residential" : [22, 0]
+      }
+    }
+  }
+]
+```
+
+The `countries` field contains a list of countries (as ISO 3166-1 alpha 2 code)
+for which the definition applies. When the field is omitted, then the
+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
+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
+file. Otherwise the import will fail with a UNIQUE INDEX constraint violation
+on import.