topic ConstructUnion method some time returning an empty geometry in ArcObjects SDK Questions

ConstructUnion method some time returning an empty geometry

ShenglinXu — Mon, 07 Jun 2010 18:07:30 GMT

I found ConstructUnion method some time returning an empty geometry even all the geometries added are not empty.
Could you help to find out why? Is it a known issue?

// Sample code
IGeometry pGeomBag = null;
IGeometryCollection geometryCollection = null;

pGeomBag = new GeometryBagClass();

pGeomBag.SpatialReference = objFeature.ShapeCopy.SpatialReference;
geometryCollection = (IGeometryCollection)pGeomBag;

//buffer each selected roads and add to geometryCollection

geometryCollection.AddGeometry(objBufferResult, ref missing, ref missing);

//Do construct union

IPolygon polygon = new PolygonClass();
ITopologicalOperator2 objTopo = polygon as ITopologicalOperator2;
if (objTopo != null)
{
objTopo.ConstructUnion((IEnumGeometry)pGeomBag);
        objTopo.IsKnownSimple_2 = false;
        objTopo.Simplify();

IGeometry objExclusion = new PolygonClass();
         objExclusion = (IGeometry)objTopo;
}

Re: ConstructUnion method some time returning an empty geometry

PascalVezina — Wed, 25 Aug 2010 14:12:49 GMT

Good day,

I am having the same issue with ConstructUnion. In my code, I try 10 times, if one of them works then I'm OK, if not I use the long "union" method instead.

Regards,

P.Vezina

Re: ConstructUnion method some time returning an empty geometry

PascalVezina — Thu, 13 Jan 2011 12:55:19 GMT

After consulting with ESRI support, it was found that limiting to 100 geometries for the ConstructUnion works. So, process by batch of 100 geometries and you'll be fine.

Regards,

P.Vezina

Re: ConstructUnion method some time returning an empty geometry

AndreaFlesca — Sat, 11 Dec 2021 15:18:09 GMT

Same problem: ConstructUnion doesn't work.
I'm developing with ArcGIs Engine SDK 10.1 for C++/Qt and I'm on Linux.
Here is my code:



    IGeometryBagPtr cGeometryBag(CLSID_GeometryBag);
    IGeometryCollectionPtr cGeometryCollection = cGeometryBag; // QI
    IGeometryPtr * cGeomVector = new IGeometryPtr[len];
    for (i = 0; i < len; i++)
    {
        IPointCollectionPtr cPointCollection(CLSID_Polygon);
        // I fill the cPointCollection object with my polygones (triangles)
        IPolygonPtr cPolygon = cPointCollection; // QI
        cGeomVector = cPolygon;
    }
    hr = cGeometryCollection -> AddGeometries((long)len, (IGeometry**)cGeomVector);
    if (FAILED(hr))
    {
        cout << "Impossible to add geometries" << std::endl;
        return;
    }
    long cGeomCount;
    hr = cGeometryCollection -> get_GeometryCount(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of geometries (1)" << std::endl;
        return;
    }
    cout << "Number of geometries (1): " << cGeomCount << std::endl;
    // in this dump I see that the number of geometries (in the GeometryCollection) is OK

    IEnumGeometryPtr cEnumGeometry = cGeometryBag; // QI
    hr = cEnumGeometry -> get_Count(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of geometries (2)" << std::endl;
        return;
    }
    cout << "Number of geometries (2): " << cGeomCount << std::endl;
    // in this dump I see that the number of geometries (in the EnumGeometry) is OK

    IPolygonPtr cResultPolygon(CLSID_Polygon);
    ITopologicalOperator2Ptr cTopoOp = cResultPolygon;
    hr = cTopoOp -> ConstructUnion(cEnumGeometry);
    if (FAILED(hr))
    {
        cout << "Impossible to do ConstructUnion" << std::endl;
        return;
    }
    IPointCollectionPtr cFinalPointCollection = cResultPolygon; // QI

    hr = cFinalPointCollection -> get_PointCount(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of points (3)" << std::endl;
        return;
    }
    cout << "Number of points (3): " << cGeomCount << std::endl;
    // In this dump the number (of points of the new geometry) is not OK, is always zero!

Where do I go wrong?
Are there examples in C++ to refer to?
The result remains the same (negative) if I change the line:

IPointCollectionPtr cFinalPointCollection = cResultPolygon; // QI

with:

IPointCollectionPtr cFinalPointCollection = cTopoOp; // QI

Any help would be appreciated.
Cheers from Italy.