Render POIs for large universities at low zoom (#1479)

This PR adapts the code used in #1457 to allow POIs to render up to z10 for very large features. I set the threshold at a very conservative value of 10% of a tile. Thus, it will render if an area POI covers at least 10% of a tile at a given zoom. Only for 'university' and  'college'.

Additionally, this consolidates a double UNION ALL on the same table with a WHERE/CASE combination, which is simpler.

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

@@ -68,20 +68,6 @@ FROM (
 
          -- etldoc: osm_poi_polygon ->  layer_poi:z12
          -- etldoc: osm_poi_polygon ->  layer_poi:z13
-         SELECT *,
-                NULL::integer AS agg_stop,
-                CASE
-                    WHEN osm_id < 0 THEN -osm_id * 10 + 4
-                    ELSE osm_id * 10 + 1
-                    END AS osm_id_hash
-         FROM osm_poi_polygon
-         WHERE geometry && bbox
-           AND zoom_level BETWEEN 12 AND 13
-           AND ((subclass = 'station' AND mapping_key = 'railway')
-             OR subclass IN ('halt', 'ferry_terminal'))
-
-         UNION ALL
-
          -- etldoc: osm_poi_polygon ->  layer_poi:z14_
          SELECT *,
                 NULL::integer AS agg_stop,
@@ -90,8 +76,21 @@ FROM (
                     ELSE osm_id * 10 + 1
                     END AS osm_id_hash
          FROM osm_poi_polygon
-         WHERE geometry && bbox
-           AND zoom_level >= 14
+         WHERE geometry && bbox AND
+           CASE
+               WHEN zoom_level >= 14 THEN TRUE
+               WHEN zoom_level >= 12 AND
+                 ((subclass = 'station' AND mapping_key = 'railway')
+                 OR subclass IN ('halt', 'ferry_terminal')) THEN TRUE 
+               WHEN zoom_level BETWEEN 10 AND 14 THEN
+                 subclass IN ('university', 'college') 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
+               ELSE FALSE END
      ) AS poi_union
 ORDER BY "rank"
 $$ LANGUAGE SQL STABLE