sql - 如何绘制与点簇面积对应的多边形？

Question

鉴于以下数据集，我正在尝试创建一个覆盖点簇最小区域的多边形：

CREATE TABLE public.points ( 
  cat integer,
  geom geometry (point, 4326)
);

INSERT INTO points VALUES 
  (1,'SRID=4326;POINT(-35.3971 -8.1366)'),
  (1,'SRID=4326;POINT(-35.388 -8.1366)'),
  (1,'SRID=4326;POINT(-35.3907 -8.1302)'),
  (1,'SRID=4326;POINT(-35.3925 -8.1366)'),
  (1,'SRID=4326;POINT(-35.3896 -8.1418)'),
  (2,'SRID=4326;POINT(-35.3996 -8.1418)'),
  (2,'SRID=4326;POINT(-35.4056 -8.1418)'),
  (2,'SRID=4326;POINT(-35.4056 -8.1542)');

我试过使用ST_Envelope，但顾名思义，它在集群周围创建了一个信封，因此比我们需要的区域大得多。

SELECT ST_Envelope(geom) FROM (
  SELECT ST_Collect(geom) AS geom
  FROM points GROUP BY cat
) mypoints

如何缩小信封以创建或多或少仅覆盖其间的点和空间的多边形？

标签： sqlpostgresqlpostgisspatial

Answer 1

要创建这样的多边形，您必须使用ST_ConvexHullor ST_ConcaveHull，例如

WITH j AS (
  SELECT ST_Collect(geom) AS geom
  FROM points GROUP BY cat
)
SELECT ST_ConvexHull(geom) FROM j 
UNION ALL SELECT geom FROM j;

注意：UNION ALL SELECT geom FROM j所有点仅用于说明目的。