How do I obtain the shape at a specific coordinate using NTS

Question

I'm using the .NET NTS library and have the code together to load a KML file. This gives me an instance of IGeometry.

Along with this data being loaded, I also have coordinates that I'd like to look up against it to see what shapes are at that point (usually will only be one). After getting the shape(s), the KML I have also includes some additional feature information which I'd then like to be able to get the values from.

My understanding of these APIs is somewhat limited and I haven't been able to find any comprehensive examples of what I need to do once I have the data loaded.

var kml = await stream.ReadToEndAsync();
var reader = new GMLReader();

geometry = reader.Read(kml);

// ...now what?

Answer 1

In your case, you can test the 'intersection' between the shape and the point like this:

IGeometry shape = ...;
IPoint point = ...;
if (shape.Intersects(point))
{
    // Do something      
}

Or maybe you could use 'within' instead or 'intersects' to test if the point is whithin the shape but not in the boundary.

Read about the DE-9IM topological model, that is used by most of GIS libraries, to know about the spatial predicate (or custom matrix) you need in each situation.

EDIT:

For the case when you have hundreds or thousands of polygons to test, you should use an spatial index.

List<IGeometry> shapes = ...;
IPoint point = ...;

// Builds the index
STRtree<IGeometry> index = new STRtree<IGeometry>();
foreach (IGeometry shape in shapes)
    index.Insert(shape.EnvelopeInternal, shape);
index.Build();

// Makes the query. The 'nearShapes' are the shapes whose envelopes contains the point.
IList<IGeometry> nearShapes = index.Query(point.EnvelopeInternal);
// Now the app only have to test the intersection against a few shapes.
foreach (IGeometry shape in nearShapes)
{
    if (shape.Intersects(point))
    {
        // Do whatever
    }
}