Add tolerance parameter to ST_Split for splitting lines by points

[michischolz]

Heho. ST_Split does not work well when splitting a line by a point. Consider the following example where the line is not split even though the blade is supposed to be a point on the line:

SELECT ST_AsText((ST_Dump(ST_Split(lin.geom, ST_Line_Interpolate_Point(lin.geom, 0.5)))).geom) AS geom
FROM (SELECT ST_GeomFromText('LINESTRING(604630.408 5792499.778,604623.849 5792500.886)') AS geom) AS lin

For the case of a line split by a point some kind of tolerance parameter would help. Until now I it is necessary first to ST_Snap your line to that interpolated point before splitting it by the point, which is not straightforward (thanks to strk for this hint). At least you could mention this hint in the documentation.

[strk]

New parameter requires minor version upgrade. michischolz: a patch for documentation is welcome

[michischolz]

Patch of documentation for ST_Split

[robe]

Okay incorporated the doco patch for 2.0 at r11251 and 2.1 at r11252

adding feature to google summer of code candidate list. http://trac.osgeo.org/postgis/wiki/GoogleSummerCode

[worthlutz]

I ran into this problem and this ticket showed up on Google. I was a bit confused trying to use ST_Snap as hinted at above. It took me a while to understand that I needed to snap the line to the point instead of the opposite. I am adding this comment to help others.

Here is my query:

SELECT
    ST_AsText( ST_LineInterpolatePoint(the_geom, 0.5) ) AS the_point_geom,
    
    ST_AsText(
        (ST_Dump(
            ST_Split( 
                ST_Snap( the_geom, ST_LineInterpolatePoint(the_geom, 0.5), 0.00000001 ), 
                ST_LineInterpolatePoint(the_geom, 0.5)
            )
        )).geom
    ) AS the_new_line_geom
    
FROM street_segments
WHERE id = 2085426

[robe]

Milestone renamed

[mwtoews]

FYI, a simple implementation of this feature is:

CREATE OR REPLACE FUNCTION ST_Split(geom1 geometry, geom2 geometry, double precision)
  RETURNS geometry AS 'SELECT ST_Split(ST_Snap($1, $2, $3), $2)'
  LANGUAGE sql IMMUTABLE STRICT COST 10;

However, it's not ideal:

SELECT ST_AsText(ST_Split('LINESTRING(0 0, 2 0)', 'MULTIPOINT(0 0, 1 0.0001)', 0.001));
-- Returns: GEOMETRYCOLLECTION(LINESTRING(1 0.0001,2 0),LINESTRING(0 0,1 0.0001))
-- Expected: GEOMETRYCOLLECTION(LINESTRING(1 0,2 0),LINESTRING(0 0,1 0))

Ideally the C-level logic would use lwgeom_snap and lwline_interpolate_points to handle a third tolerance parameter.