geotools - Coordinate conversion (EPSG:3857 to EPSG:4326) using OpenGIS/JTS too slow

I am using the Java based libraries of OpenGIS and JTS to convert coordinates from EPSG:3857 to EPSG:4326 to proceed to compute the distance using the Haversine formula. The distance features in a constraint on whether to zoom in further on a map or not.

We are using a tangible user interface and depend on the reactivity of the system. Unfortunately, the conversion is quite slow which induces even more delay than loading the map from a server. Would there be a less costly way to proceed with the conversion using local resources?

        CoordinateReferenceSystem targetCRS = CRS.decode("EPSG:4326");
        CoordinateReferenceSystem sourceCRS = CRS.decode("EPSG:3857");
        MathTransform transform = CRS.findMathTransform(sourceCRS, targetCRS, false);
        GeometryFactory geometryFactory = new GeometryFactory(new PrecisionModel(), 3857);
        Point minPoint = geometryFactory.createPoint(new Coordinate(xMin, yMin));
        Point minTargetPoint = (Point) JTS.transform(minPoint, transform);

        Point maxPoint = geometryFactory.createPoint(new Coordinate(xMax, yMax));
        PointmaxTargetPoint = (Point) JTS.transform(maxPoint, transform);

Answer

You can use the GeodeticCalculator which should be faster. Something like:

package com.envitia.spike;

import org.geotools.geometry.DirectPosition2D;
import org.geotools.referencing.CRS;
import org.geotools.referencing.GeodeticCalculator;
import org.opengis.referencing.FactoryException;

import org.opengis.referencing.NoSuchAuthorityCodeException;
import org.opengis.referencing.crs.CoordinateReferenceSystem;
import org.opengis.referencing.operation.TransformException;

public class ZoomChecker {

  CoordinateReferenceSystem sourceCRS;

  GeodeticCalculator calc;


  public ZoomChecker() throws NoSuchAuthorityCodeException, FactoryException {
    this("EPSG:3857");
  }

  public ZoomChecker(String code) throws NoSuchAuthorityCodeException, FactoryException {

    sourceCRS = CRS.decode(code);
    calc = new GeodeticCalculator(sourceCRS);

  }


  public double check(final double minX, final double minY, double maxX, double maxY)
      throws TransformException {

    calc.setStartingPosition(new DirectPosition2D(minX, minY));
    calc.setDestinationPosition(new DirectPosition2D(maxX, maxY));
    return calc.getOrthodromicDistance();
  }

  public static void main(String[] args) throws NoSuchAuthorityCodeException, FactoryException,

      TransformException {
    ZoomChecker checker = new ZoomChecker("EPSG:27700");
    double d = checker.check(0.0, 0.0, 0.0, 10000.0);
    System.out.println(d);
    d = checker.check(0.0, 0.0, 10000.0, 0.0);
    System.out.println(d);
  }
}

This gives a reasonable distance calculation of 9984m over the 10km in the test.