# [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
-54.872)");

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
increments
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
circlePoint[0][1]));
System.out.println( circlePoint[0][0] + "," +
circlePoint[0][1]);
}
} catch (IndexOutOfBoundsException ex) {

Logger.getLogger(UserOverlayFrame.class.getName()).log(Level.SEVERE,
null, ex);
} catch (IllegalArgumentException ex) {

Logger.getLogger(UserOverlayFrame.class.getName()).log(Level.SEVERE,
null, ex);
} catch (TransformException ex) {

Logger.getLogger(UserOverlayFrame.class.getName()).log(Level.SEVERE,
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
mapwidget
final GeodeticCalculator gc = new GeodeticCalculator(crs);
//set up the startPos with the objective CRS of the mapwidget
final GeneralDirectPosition startPos = new
GeneralDirectPosition(crs);

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

//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?

Thanks
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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