Descrizione

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

Questo metodo implementa la specifica SQL/MM. SQL-MM 3: 7.1.3

Questa funzione supporta il 3d e non distrugge gli z-index.

Questo metodo supporta le Curve e le Circular String.

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

Modifica: La versione 2.0.0 non funziona più con geometrie singole di stringhe multilinea. Nelle versioni precedenti di PostGIS una stringa multilinea con una sola linea avrebbe funzionato tranquillamente con questa funzione, restituendo il punto di inizio. Nella versione 2.0.0 la funzione restituisce NULL come per qualsiasi altra stringa multilinea. Il comportamento precedente non era documentato, ma le persone che presumevano di avere i dati memorizzati come LINESTRING potrebbero trovare che questi ora restituiscono il valore NULL.

Esempi

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

Start point of a 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

Si veda anche

ST_EndPoint, ST_PointN