[Qgis-developer] geos help(union polygons)

[vinayan]

Hi,

I am trying to find gaps inside a polygon layer much like the ESRI 
topology rule("must not have gaps"). I did this initially by making an 
extent polygon and then subtracting the union of all geometries from 
this extent polygon.

All this worked well and for around 100 polygons in the extent the 
performance is ok. But when the count goes up, the performance is 
drastically down. The issue, I think is because I am creating the union 
iteratively as below.

for ( it = mFeatureList1.begin(); it != FeatureListEnd; ++it )
{
g2 = it->feature.geometry();
g1 = g1->combine( g2 );
}


 From what I read from Martin Davis' 
blog(http://lin-ear-th-inking.blogspot.in/2007/11/fast-polygon-merging-in-jts-using.html), 
the cascaded polygon union improves the performance of union by a large 
ratio.

I want to use the below method that exist in Geos. But I am not sure how 
to use it.

I got this far(also pasted in pastebin http://pastebin.com/rniMLuZW),

std::vector<geos::geom::Polygon*> geomCollection;

QList<FeatureLayer>::ConstIterator FeatureListEnd = mFeatureList1.end();
for ( it = mFeatureList1.begin(); it != FeatureListEnd; ++it )
{
g1 = it->feature.geometry();
const std::string wkt = g1->exportToWkt().toStdString();
geos::geom::Polygon* geom = 
dynamic_cast<geos::geom::Polygon*>(geos::io::WKTReader().read(wkt)); 
//error is here....
geomCollection.push_back(geom);
}

geos::geom::Geometry* unionedPoly = 
geos::operation::geounion::CascadedPolygonUnion::Union(&geomCollection);


The error is,
/home/vinayan/QgisGitWorking/Quantum-GIS/src/plugins/topology/topolTest.cpp:746: 
error: cannot dynamic_cast 
‘geos::io::WKTReader().geos::io::WKTReader::read((* & wkt))’ (of type 
‘struct geos::geom::Geometry*’) to type ‘struct geos::geom::Polygon*’ 
(target is not pointer or reference to complete type)


This has more to do with my c++ knowledge than anything i guess..but if 
anyone can help where I am going wrong, it would be a good step forward 
for me. If it works well, I am sure this function would find use in 
other parts of Qgis where slow unions are involved.

Thanks in advance,
Vinayan