Fixing orientation in Net Topology Suite

Question

I have a MultiPolygon that was created by reading from a GeoJSON string.

string geoJson = getGeoJson();
var rdr = new GeoJsonReader();
var multiPolygon = rdr.Read<MultiPolygon>(geoJson);

My problem - GeoJSON recommends but does not require that the outer ring be counter-clockwise and the inner rings be clockwise.

It looks to me as if GeoJsonReader, when handed a polygon with a clockwise outside ring constructs a MultiPolygon with a clockwise outer ring.

How can I detect, in Net Topology Suite, that a ring is clockwise or counter clockwise?

Answer 1

In NTS, every object that inherits ILinarRing interface has the IsCCW property to test if it is counter clockwise.

In case of a MultiPolygon object, you have to test the shell of every internal polygon. Like this:

IMultiPolygon multiPol = rdr.Read<MultiPolygon>(geoJson);
foreach (IGeometry geom in multiPol.Geometries)
{
    IPolygon pol = (IPolygon)geom;
    if (pol.Shell.IsCCW)
    {
        // Shell is counter clockwise
    }
    else
    {
        // Shell is clockwise
        IGeometry ccwShell = pol.Shell.Reverse();
    }
}

PD: Old question, maybe usefull for someone...