Name

ST_LineInterpolatePoint — Returns a point interpolated along a line at a fractional location.

Synopsis

geometry ST_LineInterpolatePoint(geometry a_linestring, float8 a_fraction);

Description

Returns a point interpolated along a line at a fractional location. First argument must be a LINESTRING. Second argument is a float between 0 and 1 representing the fraction of total linestring length where the point is to be located. The Z and M values are interpolated if present.

See ST_LineLocatePoint for computing the line location nearest to a Point.

[Note]

This function computes points in 2D and then interpolates values for Z and M, while ST_3DLineInterpolatePoint computes points in 3D and only interpolates the M value.

[Note]

Since release 1.1.1 this function also interpolates M and Z values (when present), while prior releases set them to 0.0.

Availability: 0.8.2, Z and M supported added in 1.1.1

Changed: 2.1.0. Up to 2.0.x this was called ST_Line_Interpolate_Point.

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

Examples

A linestring with the interpolated point at 20% position (0.20)

-- The point 20% along a line

SELECT ST_AsEWKT(
    ST_LineInterpolatePoint(
        'LINESTRING(25 50, 100 125, 150 190)',
        0.2 ));

   st_asewkt
----------------
 POINT(51.5974135047432 76.5974135047432)

The mid-point of a 3D line:

SELECT ST_AsEWKT(
    ST_LineInterpolatePoint('
        LINESTRING(1 2 3, 4 5 6, 6 7 8)',
        0.5 ));

	st_asewkt
--------------------
 POINT(3.5 4.5 5.5)

The closest point on a line to a point:

SELECT ST_AsText(
    ST_LineInterpolatePoint( line.geom,
        ST_LineLocatePoint( line.geom, 'POINT(4 3)')))
FROM (SELECT ST_GeomFromText('LINESTRING(1 2, 4 5, 6 7)') As geom) AS line;

 st_astext
------------
 POINT(3 4)

See Also

ST_LineInterpolatePoints, ST_3DLineInterpolatePoint, ST_LineLocatePoint