Descrizione

Returns the M coordinate of the first point of a LINESTRING, CIRCULARLINESTRING, or NURBSCURVE geometry. Returns NULL if the input geometry does not have an M dimension or is not a LINESTRING, CIRCULARLINESTRING, or NURBSCURVE.

Questo metodo supporta le Curve e le Circular String.

Esempi

Start M value of a LineString

SELECT ST_StartM('LINESTRING M(0 0 10, 1 1 20, 2 0 30)'::geometry);
 st_startm
-----------
        10

Start M value of a CircularString

SELECT ST_StartM('CIRCULARSTRING M(0 0 5, 1 1 10, 2 0 15)'::geometry);
 st_startm
-----------
         5

Start M value of a NURBSCurve

SELECT ST_StartM('NURBSCURVE M(2, (0 0 100, 1 1 200, 2 0 300))'::geometry);
 st_startm
-----------
       100

Geometry without M dimension returns NULL

SELECT ST_StartM('LINESTRING(0 0, 1 1)'::geometry) IS NULL AS is_null;
 is_null
---------
 t

Si veda anche

ST_EndM, ST_SetStartM, ST_StartPoint