Allow big POIs at low zoom

layers/poi/mapping.yaml

@@ -327,6 +327,8 @@ def_poi_fields: &poi_fields
     type: id
   - name: geometry
     type: geometry
+  - name: area
+    type: area
   - name: name
     key: name
     type: string

layers/poi/poi.sql

@@ -91,7 +91,16 @@ FROM (
                     END AS osm_id_hash
          FROM osm_poi_polygon
          WHERE geometry && bbox
-           AND zoom_level >= 14
+           AND (
+            zoom_level >= 14 OR
+            zoom_level >= 10 AND
+              POWER(4,zoom_level)
+              -- Compute percentage of the earth's surface covered by this feature (approximately)
+              -- The constant below is 111,842^2 * 180 * 180, where 111,842 is the length of one degree of latitude at the equator in meters.
+              * area / (405279708033600 * COS(ST_Y(ST_Transform(geometry,4326))*PI()/180))
+              -- Match features that are at least 10% of a tile at this zoom
+              > 0.10
+           )
      ) AS poi_union
 ORDER BY "rank"
 $$ LANGUAGE SQL STABLE