Descripción

Returns the first point of a LINESTRING, CIRCULARLINESTRING, or NURBSCURVE geometry as a POINT. For other geometries, returns the first point in coordinate order.

This method implements the SQL/MM specification. SQL-MM 3: 7.1.3

This function supports 3d and will not drop the z-index.

This method supports Circular Strings and Curves.

Enhanced: 3.2.0 returns a point for all geometries. Prior behavior returns NULLs if input was not a LineString.

Cambiado: 2.0.0 ya no funciona con multilinestrings de geometrías simples. En versiones anteriores de PostGIS -- una linea simple multilinestring funciona sin problemas con esta función y devuelve el punto inicial. En la version 2.0.0 simplemente devuelve NULL como con cualquier multilinestring. La antigua version era una función sin documentar, pero la gente que asumía que tenia sus datos almacenados en LINESTRING pueden experimentar este comportamiento ahora de resultado NULL en la version 2.0.

Ejemplos

Start point of a LineString

SELECT ST_StartPoint('LINESTRING(0 1,0 2)'::geometry);

POINT(0 1)

01010000000000000000000000000000000000F03F

Figure

Start point of a Point is the Point itself

SELECT ST_StartPoint('POINT(0 1)'::geometry);

POINT(0 1)

01010000000000000000000000000000000000F03F

Figure

Start point of a 3D LineString

SELECT ST_StartPoint('LINESTRING(0 1 1,0 2 2)'::geometry);

POINT(0 1 1)

01010000800000000000000000000000000000F03F000000000000F03F

Figure

Devuelve el número de puntos en un valor ST_LineString o ST_CircularString.

SELECT ST_StartPoint('CIRCULARSTRING(5 2,-3 2,-2 1,-4 2,6 3)'::geometry);

POINT(5 2)

010100000000000000000014400000000000000040

Figure

Start point of a NURBSCurve

SELECT ST_StartPoint('NURBSCURVE(2, (0 0, 1 1, 2 0))'::geometry);

POINT(0 0)

010100000000000000000000000000000000000000

Figure

Build edge node references from the start and end points of a LineString table. The example uses exact endpoint equality, so linework with nearly coincident endpoints should be snapped or otherwise cleaned first.

WITH roads(road_id, geom) AS (
  VALUES
    (1, 'LINESTRING(0 0, 1 0)'::geometry),
    (2, 'LINESTRING(1 0, 1 1)'::geometry),
    (3, 'LINESTRING(1 0, 2 0)'::geometry)
),
endpoints AS (
  SELECT road_id, 'from' AS end_name, ST_StartPoint(geom) AS geom,
         ST_AsEWKB(ST_StartPoint(geom)) AS endpoint_key
  FROM roads
  UNION ALL
  SELECT road_id, 'to' AS end_name, ST_EndPoint(geom) AS geom,
         ST_AsEWKB(ST_EndPoint(geom)) AS endpoint_key
  FROM roads
),
nodes AS (
  SELECT dense_rank() OVER (ORDER BY ST_X(geom), ST_Y(geom), endpoint_key) AS node_id,
         endpoint_key
  FROM (
    SELECT DISTINCT ON (endpoint_key) endpoint_key, geom
    FROM endpoints
    ORDER BY endpoint_key
  ) AS distinct_endpoints
)
SELECT e.road_id,
       max(n.node_id) FILTER (WHERE e.end_name = 'from') AS from_node,
       max(n.node_id) FILTER (WHERE e.end_name = 'to') AS to_node
FROM endpoints AS e
JOIN nodes AS n USING (endpoint_key)
GROUP BY e.road_id
ORDER BY e.road_id;

road_id | from_node | to_node
---------+-----------+---------
     1 |         1 |       2
     2 |         2 |       3
     3 |         2 |       4

Figure

Ver también

ST_EndPoint, ST_PointN