[geos-devel] [GEOS] #758: LineStringSnapper snaps in wrong order in last segment of closed linestring

[GEOS]

#758: LineStringSnapper snaps in wrong order in last segment of closed linestring
------------------------+-------------------------------
 Reporter:  maxbo       |      Owner:  geos-devel@…
     Type:  defect      |     Status:  new
 Priority:  minor       |  Milestone:  3.5.1
Component:  Default     |    Version:  3.5.0
 Severity:  Unassigned  |   Keywords:  LineStringSnapper
------------------------+-------------------------------
 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);

         char* wkt_c = GEOSWKTWriter_write(w_, geom3_);
         std::string out(wkt_c);
         free(wkt_c);

         ensure_equals(out, "LINESTRING (0 0, 5 9, 10 0, 1 0.5, 0 0)");
     }

 Thanks,

 Max

--
Ticket URL: <https://trac.osgeo.org/geos/ticket/758>
GEOS <http://trac.osgeo.org/geos>
GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite (JTS).