[GIS] How to tell if a line intersects a polygon not only at endpoints

jts-topology-suitenettopologysuite

How do I do this in .net topology suite or jts? Please see image.

Any other library that can I run on .NET is fine, or even a general algorithm if there is no option.

For example this answer is only for endpoints:
Check if line crosses a polygon

polygon-line-intersect

Right now I'm randomly experimenting and if i intersect and check

if (intersection.Boundary.ToString().Contains("EMPTY"))

it seems to work, but… yeah no idea why or if it's all cases.

Best Answer

From your drawings and the comments I think your requirements are that the line

intersects with the interior of the polygon
does not just touch the boundary of the polygon

Just combine the requirements. This is the method using JTS that should answer your question:

public static boolean lineReallyIntersectsPolygon(LineString ls, Polygon p){
    if(ls.intersects(p) && !ls.touches(p)){
        return true;
    } else {
        return false;
    }
}

Related Solutions

Polygon – How to Check If Line Crosses a Polygon

You need to understand the definition of the spatial predicates. What you're really want is to know if a line traverses the polygon (and not simply cross it)

Using the JTS Topology Suite, you can see that in both cases the predicates are the same: the two lines intersects and crosses the polygon:

enter image description here

and

enter image description here

With the JTS Topology Suite, you can use the Dimensionally Extended nine-Intersection Model (DE-9IM) and the Clementini Matrix:

In the first case it is
1 F 2
0 F 1

And in the second, it is
1 0 2
0 F 1

What does that really mean:

in the second case the result of the intersection of the boundary of the polygon and the boundary of the line has a dimension of 0 (must obligatorily be one Point, and only one)
in the first case the result of the intersection of the boundary of the polygon and the boundary of the line has a dimension of F (is not important, 2 Point here)

Therefore you are able to discriminate between the two cases.

line.intersection(LinearRing(polygon)) = 1 Point

line.intersection(LinearRing(polygon)) = 2 Points

New: if the polygon has holes:

You need to understand the topological definitions of interior, exterior and boundary of a geometry: this is fundamental if you work with JSTS, JTS or GEOS.

1) the polygon

enter image description here

2) the boundary of the polygon in red (LinearRing)

enter image description here !

3) The result of the intersection of the line and the boundary of the polygon because the holes lie within the interior of the polygon

enter image description here

And the resulting DE-9IM matrix change:

enter image description here

[GIS] How two add line from point A to B

The CreateLineString method only takes one argument - an array of Coordinate (strictly speaking an IEnumerable<ICoordinate> or an ICoordinateSequence). You can see this in the examples:

//Add a LineString with 2 Points
GeoAPI.Geometries.Coordinate[] points = new GeoAPI.Geometries.Coordinate 
    {
        new GeoAPI.Geometries.Coordinate(10, 10), 
        new GeoAPI.Geometries.Coordinate(20, 20), 
    };

geomColl.Add(gf.CreateLineString(points));

Note that there are some API differences across versions, so next time, posting exact version information might be useful.

Best Answer

Related Solutions

Polygon – How to Check If Line Crosses a Polygon

New: if the polygon has holes:

[GIS] How two add line from point A to B

Related Question