Beskrivning

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

Denna metod implementerar SQL/MM-specifikationen. SQL-MM 3: 7.1.3

Denna funktion stöder 3d och kommer inte att tappa z-index.

Denna metod stöder cirkulära strängar och kurvor.

Förbättrad: 3.2.0 returnerar en punkt för alla geometrier. Tidigare beteende returnerade NULL om indata inte var en LineString.

Ändrad: 2.0.0 fungerar inte längre med MultiLineStrings med en geometri. I äldre versioner av PostGIS skulle en MultiLineString med en enda rad fungera bra med den här funktionen och returnera startpunkten. I 2.0.0 returnerar den bara NULL som vilken annan MultiLineString som helst. Det gamla beteendet var en odokumenterad funktion, men personer som antog att de hade sina data lagrade som LINESTRING kan uppleva att dessa returnerar NULL i 2.0.0.

Exempel

Startpunkt för en 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

Startpunkt för en 3D-linjeString

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

POINT(0 1 1)

01010000800000000000000000000000000000F03F000000000000F03F

Figure

Startpunkt för en 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);

PUNKT(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

Se även

ST_EndPoint, ST_PointN