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#758 closed defect (fixed)

LineStringSnapper snaps in wrong order in last segment of closed linestring

Reported by:	maxbo	Owned by:	geos-devel@…
Priority:	minor	Milestone:	3.6.1
Component:	Default	Version:	3.4.2
Severity:	Unassigned	Keywords:	LineStringSnapper
Cc:

Description

st_snap(linestring, geometry) behaves not always as expected for closed linestrings in the last segment. This seems to be a bug in the geos LineStringSnapper class:

(using (POSTGIS="2.1.5 r13152" GEOS="3.4.2-CAPI-1.8.2 r3924" PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 1.11.1, released 2014/09/24 GDAL_DATA not found" LIBXML="2.7.8" LIBJSON="UNKNOWN" TOPOLOGY RASTER)

The following example should demonstrate the problem:

SELECT (du).path[1] as vertex_nr, st_astext((du).geom) FROM ( SELECT st_dumppoints(st_snap(a, b, 2)) AS du FROM (SELECT 'LINESTRING (1 1, 5 9, 9 1, 1 1)'::geometry AS a, 'MULTIPOINT(0 0, 10 0, 1 0.5)'::geometry AS b ) c )d;

I expected that POINT(1 0.5) would be inserted into the last segment of the LINESTRING at position 4. This point is acutally inserted, but at a wrong position (Position 3), resulting in an "ugly" geometry.

1;POINT(0 0) 2;POINT(5 9) 3;POINT(1 0.5) 4;POINT(10 0) 5;POINT(0 0)

This behaviour only occures for a closed linestring. Opening the linestring SELECT (du).path[1] as vertex_nr, st_astext((du).geom) FROM ( SELECT st_dumppoints(st_snap(a, b, 2)) AS du FROM (SELECT 'LINESTRING (0.9 0.9, 5 9, 9 1, 1 1)'::geometry AS a, 'MULTIPOINT(0 0, 10 0, 1 0.5)'::geometry AS b ) c )d;

will take POINT(1, .5) as endpoint.

1;POINT(0 0) 2;POINT(5 9) 3;POINT(10 0) 4;POINT(1 0.5)

Shifting the start- and endpoint of the linestring

SELECT (du).path[1] as vertex_nr, st_astext((du).geom) FROM ( SELECT st_dumppoints(st_snap(a, b, 2)) AS du FROM (SELECT 'LINESTRING (5 9, 9 1, 1 1, 5 9)'::geometry AS a, 'MULTIPOINT(0 0, 10 0, 1 0.5)'::geometry AS b ) c )d;

results in a correct geometry, where POINT(1, .5) is inserted at position 3 between POINT(10 0) and POINT(0 0)

1;POINT(5 9) 2;POINT(10 0) 3;POINT(1 0.5) 4;POINT(0 0) 5;POINT(5 9)

Here a test for GEOSSnapTest.cpp

/ Test snapping of last segment of closed ring to snap points

{

geom1_ = GEOSGeomFromWKT("LINESTRING (1 1, 5 9, 9 1, 1 1)"); geom2_ = GEOSGeomFromWKT("MULTIPOINT(0 0, 10 0, 1 0.5)"); geom3_ = GEOSSnap(geom1_, geom2_, 2);