[geotk] Circle Polygon

theuns theunsheydenrych at gmail.com
Thu Sep 17 06:04:38 EDT 2009

HI Martin

I tried to calculate the circle with the GeodeticCalculator with
EPSG:3395 and with WGS84 Geographic projection.

Attached is the results for the tests.

The results does not look correctly, for the CRS_3395 i excpexted a egg
shaped circle ,and for the WGS84 a "round" circle.

For the test a took a world shapefile and the shapefile is in geographic
coordinates and added a circle at the southern tip of south america,
will see the shapefile circle also rendered in the attached images.
I mailed this shapefile to Johann, if you would like to use it, or i can
mail it again to you if you would like to have it.

I then used a point more or less in the middle of the shapefile circle ,
to generate a circle of 100km radius in more or less the same place.

Below is the code i used to to this:

The radius value i used is 100000

//Centre point of circle used for EPSG:3395
Geometry centrePoint = StaticGeometry.geomFromWKT("POINT
(-7144442.991431108 -7277982.720580169)");

//Centre point of circle used for WGS84 Geographic
//Geometry centrePoint = StaticGeometry.geomFromWKT("POINT (-64.174

protected Geometry getCircle(Geometry centrePoint, double radius,
CoordinateReferenceSystem objectiveCRS) {
        Geometry resultCircle = null;
        final Point jtsPoint = (Point) centrePoint;
        double x = jtsPoint.getCoordinate().x;
        double y = jtsPoint.getCoordinate().y;

        //List for result coordinates
        List<Coordinate> circleCoords = new ArrayList<Coordinate>();
        //double array for buffer result
        double[][] circlePoint;
        try {
            //Work from 180 degrees down to -180 degrees in 10 degree
            for (int azm = 180; azm >= -180; azm -= 10) {
                //get the point on the circle ring for a distance from
the centre point at a certain angle
                circlePoint = buffer(x, y, objectiveCRS, radius, azm);
                //make a coordinate of the result and add it to the
coords list
                circleCoords.add(new Coordinate(circlePoint[0][0],
                System.out.println( circlePoint[0][0] + "," +
        } catch (IndexOutOfBoundsException ex) {

null, ex);
        } catch (IllegalArgumentException ex) {

null, ex);
        } catch (TransformException ex) {

null, ex);

        //Create a polygon from the coordinates list
        Coordinate[] ringCoords = circleCoords.toArray(new
Coordinate[circleCoords.size() + 1]);
        ringCoords[circleCoords.size()] = circleCoords.get(0);
        GeometryFactory GF = new GeometryFactory();
        LinearRing ring = GF.createLinearRing(ringCoords);
        resultCircle = GF.createPolygon(ring, new LinearRing[0]);

        return resultCircle;

private double[][] buffer(double x, double y, CoordinateReferenceSystem
crs, double distance, double angle) throws IndexOutOfBoundsException,
IllegalArgumentException, TransformException {
        //create array for the result
        double[][] result = new double[1][2];
        //set up the GeodeticCalculator with the objective CRS of the
        final GeodeticCalculator gc = new GeodeticCalculator(crs);
        //set up the startPos with the objective CRS of the mapwidget
        final GeneralDirectPosition startPos = new

        //Populate the StartingPosition in the GeodeticCalculator,
        //in this case its the centre point of the circle
        startPos.ordinates[0] = x;
        startPos.ordinates[1] = y;

        //Set the direction and distance for the point to be computed
        gc.setDirection(angle, distance);
        //Get the computed result
        DirectPosition destPos = gc.getDestinationPosition();
        //Populate the result array, with computed x,y values
        result[0][0] = destPos.getOrdinate(0);
        result[0][1] = destPos.getOrdinate(1);
        return result;

I think the problem is in the GeodedicCalculator when converting the
values back to the supplied CRS, or i just dont use the
GeodedicCalculator correctly?

Theuns Heydenrych

On Wed, 2009-09-16 at 21:23 +0200, Martin Desruisseaux wrote:
> theuns a écrit :
> > The suggestion is now to use the GeodedicCalculator to work out the
> > values for the circle, for a certain CRS, this makes more sense to me,
> > since the geodedicCalculator will take in account the CRS.
> > 
> > Do you agree?
> Yes, it is the best approach I can imagine right now.
> 	Regards,
> 		Martin
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