How to Swap Coordinates of jts.geom.Geometry object from Lat, Long to Long,Lat in JTS

Question

I have a geometry object of type (com.vividsolutions.jts.geom.Geometry). it is currently in latitude, longitude form and I'd like to flip the coordinates so that its longitude latitude so that I can have it in GeoJSON format for mongodb.

My constraints that I am seeing are: a) the input that I would like to flip coordinates for is a Geometry object. b) The Geometry object will be either a Polygon type or Multipolygon. c) I would like to flip the coordinates before the type cast to Polygon/multipolygon

I have tried geo.reverse() but it does not work.

As well, I have tried using: CRSAuthorityFactory factory = CRS.getAuthorityFactory(true); CoordinateReferenceSystem crs = factory.createCoordinateReferenceSystem("EPSG:4326");

And another option and I did not see it work.

Thanks!

Answer 1

You can use a CoordinateFilter to invert all x and y values in a given geometry.

private static class InvertCoordinateFilter implements CoordinateFilter {
    public void filter(Coordinate coord) {
        double oldX = coord.x;
        coord.x = coord.y;
        coord.y = oldX;
    }
}

Then apply the filter thusly:

// Invert Coordinates since GeoJSON likes them that way
myGeometryObj.apply(new InvertCoordinateFilter());

Answer 2

The solution is that the Geometry.getCoordinates() gives a Coordinate[] array which is live. Therefore, I could use the following:

Where myGeometryObject is a Geometry object:

Coordinate[] original = myGeometryobject.getCoordinates();
for(int i = 0; i < original.length; i++){
    Double swapValue = original[i].x;
    original[i].x = original[i].y;
    original[i].y = swapValue;
}

The changes to the Coordinate objects will affect the underlying Geometry permanently.

Answer 3

As important addendum to the existing answers: You should always call yourGeometry.geometryChanged() after you changed a Geometry's geometry!

See http://www.vividsolutions.com/jts/javadoc/com/vividsolutions/jts/geom/Geometry.html#getCoordinates%28%29 and http://www.vividsolutions.com/jts/javadoc/com/vividsolutions/jts/geom/Geometry.html#geometryChanged%28%29

Answer 4

One potential solution to this is extending the class to provide an additional function that either outputs the data you need in some convenient way:

public Coordinate[] getReversedCoordinates(){

  Coordinate[] original = this.getCoordinates();
  Coordinate[] ret = new Coordinate[original.length];

  for(int i =0; i<original.length; i++){
      ret[i] = new Coordinate( original[i].x , original[i].y );
  }

  return ret;

}

Alternately you could alter the interpretation of the data. It's a little harder for me to give you a code snippet for that as I'm not sure how you're using the information specifically.

EDIT:

Once you have the reversed coordinates,you can create a duplicate Geometry of type linear ring. A means of doing this is to use your factory to use your geometry factory:

GeometryFactory gf = //However this was instantiated;
Coordinate[] reversedCoordinates = getReversedCoordinates();
gf.createLinearRing(reversedCoordinates);

Happy coding and leave a comment if you have any questions!

Answer 5

As the JTS javadocs suggest, it is best to use a CoordinateSequenceFilter to change the coordinates of a geometry in place.

/**
 * Swaps the XY coordinates of a geometry.
 */
public void swapCoordinates(Geometry g) {
    g.apply(new CoordinateSequenceFilter() {

        @Override
        public void filter(CoordinateSequence seq, int i) {
            double oldX = seq.getCoordinate(i).x;
            seq.getCoordinate(i).x = seq.getCoordinate(i).y;
            seq.getCoordinate(i).y = oldX;
        }

        @Override
        public boolean isGeometryChanged() {
            return true;
        }

        @Override
        public boolean isDone() {
            return false;
        }
    });
}

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_{NOTE: there are nasty cases where Coordinate object you get access to is a copy of what is internally stored (eg. see PackedCoordinateSequence). In that case, as suggested in the javadocs, you must use the provided setters, that is setX() setY().}

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_{There are also even nastier cases where there is simply no way to change the coordinates in place, eg. when fetching a geometry from the DB with PostGIS where this Geolatte PackedPositionSequence sequence implementation just won't let you modify the coordinates (afaik).}