Union of two Polygons results in GeometryCollection

[brownjt]

#include <geos/io/WKBReader.h>
#include <sstream>
#include <cassert>

int main()
{
    std::string a = 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    std::string b = 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    std::istringstream sa(a);
    std::istringstream sb(b);
    geos::io::WKBReader reader;
    
    std::unique_ptr<geos::geom::Geometry> ga = reader.readHEX(sa);
    assert(ga->isValid());
    assert(ga->getGeometryTypeId() == geos::geom::GEOS_POLYGON);

    std::unique_ptr<geos::geom::Geometry> gb = reader.readHEX(sb);
    assert(gb->isValid());
    assert(gb->getGeometryTypeId() == geos::geom::GEOS_POLYGON);

    std::unique_ptr<geos::geom::Geometry> gc = ga->Union(gb.get());
    assert(gc->isValid());
    geos::geom::GeometryTypeId type = gc->getGeometryTypeId();
    std::cout << type << std::endl;
    assert(type == geos::geom::GEOS_POLYGON);
}

Expected: result geometry is a Polygon

Actual: result is a GeometryCollection which contains the expected Polygon as well as an extraneous LineString consisting of two points.

[pramsey]

Have a look at OverlayNG.setStrictMode(). If you turn on strict mode, the results of overlays will only include components of a single dimensionality. Unfortunately (?) the results of the standard methods on Geometry have to match up to the old behaviour for legacy reasons, so strict mode is turned off by default.

[brownjt]

Replying to pramsey:

Have a look at OverlayNG.setStrictMode(). If you turn on strict mode, the results of overlays will only include components of a single dimensionality. Unfortunately (?) the results of the standard methods on Geometry have to match up to the old behaviour for legacy reasons, so strict mode is turned off by default.

I don't understand how this is invalid. The issue is not with the dimensionality of the result. There should be only one resulting geometry from the union not two. It has been some time since I looked at this but I believe the extra line string actually has duplicate points that are already present on the resulting polygon. Why is it generating these extra points?

[mdavis]

The result is reflecting the fact that the vertices of the two polygons are slightly different to each other. In fact polygon A has a vertex which creates a triangle slightly outside polygon B. The difference is so small that the triangle collapses to a line. (See attached image).

The overlay algorithm is able to compute geometry for these situations, and optionally provide it (as a way of seeing where collapses have occurred). As Paul said, for backwards compatibility reasons by default the collapses are returned in the result.

The reason the ticket is marked as invalid is because this is expected behaviour.

[mdavis]

Shows Polygon A extending slightly outside Polygon B, resulting in a topology collapse during union