How to define if a determinate point is inside a region lat, long?

Question

I have a region defined by a set List of geopoints and I need to know if a coordinate is inside this region

public class Region{
    List<Coordinate> boundary;

}

public class Coordinate{

    private double latitude;
    private double longitude;

}

public static boolean isInsideRegion(Region region, Coordinate coordinate){


}

Answer 1

You can apply a Point in polygon algorithm from the Computational Geometry set of problems.

There are four algorithms written in C by Paul Bourke, you can see the code here. There is an adaptation to Java in a Processing Forum, just in case you can't use Java7:

public class RegionUtil {

    boolean coordinateInRegion(Region region, Coordinate coord) {
        int i, j;
        boolean isInside = false;
        //create an array of coordinates from the region boundary list
        Coordinate[] verts = (Coordinate)region.getBoundary().toArray(new Coordinate[region.size()]);
        int sides = verts.length;
        for (i = 0, j = sides - 1; i < sides; j = i++) {
            //verifying if your coordinate is inside your region
            if (
                (
                 (
                  (verts[i].getLongitude() <= coord.getLongitude()) && (coord.getLongitude() < verts[j].getLongitude())
                 ) || (
                  (verts[j].getLongitude() <= coord.getLongitude()) && (coord.getLongitude() < verts[i].getLongitude())
                 )
                ) &&
                (coord.getLatitude() < (verts[j].getLatitude() - verts[i].getLatitude()) * (coord.getLongitude() - verts[i].getLongitude()) / (verts[j].getLongitude() - verts[i].getLongitude()) + verts[i].getLatitude())
               ) {
                isInside = !isInside;
            }
        }
        return isInside;
    }
}

Answer 2

Use Path2D to construct the region boundary shape. Then, create an Area using the Path2D and you can query contains quickly to determine whether your points are contained in the area. :-)

/* assuming a non-zero winding rule */
final Path2D boundary = new Path2D.Double();
/* initialize the boundary using moveTo, lineTo, quadTo, etc. */
final Area area = new Area(boundary);
...
/* test for whether a point is inside */
if (area.contains(...)) {
  ...
}

Note: there is little reason to roll your own Region and Coordinate classes for what the Java geometry classes provide. I suggest you abandon Coordinate (which is technically a misnomer as it's actually a pair of graticular coordinates) in favor of Point2D.

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Note that there is a Polygon class, though it is tailored towards actual use for graphics and a relic of the past. It only supports int coordinates, which likely won't do you any good when using geopoints!