How to calculate centroid of a polygon defined by a list of longitude/latitude points? [duplicate]

Question

I've imported some polygons (ways) from OpenStreetMap and now want to calculate the centroids of them.

For this purpose I wrote following code:

    final List<Point2D.Double> points = getPoints();
    final Point2D.Double actualCentroid = calculateCentroidOurWay(points);

    LOGGER.debug("Centroid, x: " + actualCentroid.x);
    LOGGER.debug("Centroid, y: " + actualCentroid.y);

private Point.Double calculateCentroidOurWay(final List<Point2D.Double> aPoints) {
    final Building objectUnderTest = new Building();

    objectUnderTest.setPolygon(new Polygon(aPoints));

    return objectUnderTest.calculateCentroid();
}

@Override
public Point2D.Double calculateCentroid() {
    double x = 0.;
    double y = 0.;
    double area = 0.;

    for (int i = 0; i < polygon.getPoints().size() - 1; i++) {
        final Point2D.Double point = polygon.getPoints().get(i);
        final Point2D.Double pointN = polygon.getPoints().get(i + 1);

        final double temp = point.getX() * pointN.getY() - pointN.getX() * point
                .getY();
        x += (point.getX() + pointN.getX()) * temp;

        y += (point.getY() + pointN.getY()) * temp;

        area += temp;
    }

    area /= 2.;

    x *= 1 / 6. * area;
    y *= 1 / 6. * area;

    return new Point2D.Double(x, y);
}


private List<Point.Double> getPoints() {
    final List<Point.Double> points = new LinkedList<>();

    points.add(new Point.Double(132.1764353, 43.3539477));
    points.add(new Point.Double(132.1771393, 43.3539409));
    points.add(new Point.Double(132.177145, 43.3542587));
    points.add(new Point.Double(132.1768224, 43.3542618));
    points.add(new Point.Double(132.1768183, 43.3540332));
    points.add(new Point.Double(132.1766516, 43.3540348));
    points.add(new Point.Double(132.1766564, 43.3542949));
    points.add(new Point.Double(132.176566, 43.3542957));
    points.add(new Point.Double(132.1765705, 43.3545391));
    points.add(new Point.Double(132.1769718, 43.3545353));
    points.add(new Point.Double(132.1769691, 43.3543859));
    points.add(new Point.Double(132.177131, 43.3543843));
    points.add(new Point.Double(132.1771321, 43.3544446));
    points.add(new Point.Double(132.177341, 43.3544426));
    points.add(new Point.Double(132.1773296, 43.3538152));
    points.add(new Point.Double(132.176433, 43.3538238));
    points.add(new Point.Double(132.1764353, 43.3539477));
    return points;
}

The result of the calculation is

Centroid, x: 1.8787362164666136E-11
Centroid, y: 6.162273026564668E-12

which is obviously wrong (it's not a latitude/longitude pair).

When I use Java Topology Suite, it returns same results.

private Coordinate calculateJtsCentroid(final List<Point2D.Double> aPoints) {
    final CentroidPoint centroidPoint = new CentroidPoint();

    for (final Point.Double curPoint : aPoints) {
        centroidPoint.add(new Coordinate(curPoint.x, curPoint.y));
    }

    return centroidPoint.getCentroid();
}

How should I change my code in order to get the centroid of the polygon as a point identified by latitude and longitude?

Answer 1

Since you have the latitudes and longitudes of each point you can use the Finite Set of Points method indicated on Wikipedia. I think something like this might work.

public Point2D.Double calculateCentroid() {
    double x = 0.;
    double y = 0.;
    int pointCount = polygon.getPoints().size();
    for (int i = 0;i < pointCount - 1;i++){
        final Point2D.Double point = polygon.getPoints().get(i);
        x += point.getX();
        y += point.getY();
    }

    x = x/pointCount;
    y = y/pointCount;

    return new Point2D.Double(x, y);
}

Basically you just need to average the x and y separately, then create a new point with those averages as the coordinates.