如何在 PostgreSQL/PostGIS 中获得 3 点三角形的面积？

Question

I want to compute the area of a triangle with 3 points with QGIS or PostgreSQL/PostGIS. The points are saved in a geometry column. I have no clue how to do it...

Answer 1

Try ST_Area , eg

WITH j AS (
SELECT 
  'POLYGON((7.23 49.95,10.04 52.91,11.62 49.89,7.23 49.95))'::geometry AS poly) 
SELECT ST_Area(poly) FROM j;

      st_area      
-------------------
 6.581499999999988
(1 Zeile)

Using geometry parameters it ..

Returns the area of a polygonal geometry. For geometry types a 2D Cartesian (planar) area is computed, with units specified by the SRID

and using geography ..

For geography types by default area is determined on a spheroid with units in square meters.

WITH j AS (
SELECT 
  'POLYGON((7.23 49.95,10.04 52.91,11.62 49.89,7.23 49.95))'::geography AS poly) 
SELECT ST_Area(poly) FROM j;

      st_area      
-------------------
 52245336562.55078
(1 Zeile)

EDIT : To create a polygon based on three points use ST_MakePolygon and ST_MakeLine (see comments):

WITH j (p1,p2,p3) AS ( 
  VALUES ('POINT(7.23 49.95)','POINT(10.04 52.91)','POINT(11.62 49.89)'))
SELECT 
  ST_AsText(ST_MakePolygon(ST_MakeLine(ARRAY[p1, p2, p3, p1])),2)
FROM j;

                        st_astext                         
----------------------------------------------------------
 POLYGON((7.23 49.95,10.04 52.91,11.62 49.89,7.23 49.95))
(1 Zeile)