PostGIS
Name
ST_LineExtend — Returns a line with the last and first segments extended the specified distance(s).
Synopsis
geometry +`*`+ST_LineExtend*(`geometry `line, float
distance_forward, float distance_backward=0.0`)`;
Description
Returns a line with the last and first segments extended the specified distance(s). Distance of zero carries out no extension. Only non-negative distances are allowed. The first (and last) two distinct points in a line are used to determine the direction of projection, duplicate points are ignored.
Availability: 3.4.0
Example: Extends a line 5 units forward and 6 units backward
SELECT ST_AsText(ST_LineExtend('LINESTRING(0 0, 0 10)'::geometry, 5, 6));
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LINESTRING(0 -6,0 0,0 10,0 15)