Hi,
In my java app I need to calculate coordinates for the google tile
coordinate system.
I basically have two use cases :
1. get the XY pixel coordinates at zoom for a given latitude /
longitude
2. get the latitude / longitude coordinates for a given XY pixel
coordinate at a given zoom
Here's the code I'm currently using :
//returns a point that is a google pixel reference for the particular
lat/lng and zoom
public static Point latLngToAbsolutePixelCoordsAtZoom(double lat,
double lon, int zoom, int tileSize){
//normalize
// first convert to Mercator projection
// first convert the lat lon to mercator coordintes.
if (lon > 180)
{
lon -= 360;
}
lon /= 360;
lon += 0.5;
lat = 0.5 - ((Math.log(Math.tan((Math.PI / 4) + ((0.5 *
Math.PI * lat) / 180))) / Math.PI) / 2.0);
//now that we have the normalized coords
double scale = (1 << zoom) * tileSize;
Point ret = new Point((int)( lon *scale), (int)(lat* scale));
return ret;
}
//PointD is a XY point using java double primitive
public static PointD pixelToLatLong(int pixelX, int pixelY, int zoom)
{
double longitude = (((double)pixelX * 360) / (256 * Math.pow
(2, zoom))) - 180;
double efactor = Math.exp((0.5 - (double)pixelY / 256 /
Math.pow(2, zoom)) * 4 * Math.PI);
double latitude = Math.asin((efactor - 1) / (efactor + 1)) *
180 / Math.PI;
return new PointD(longitude, latitude);
}
You can test this code as follow :
private void runTest(){
System.out.println("initial values = lat = 46.20181075433888 "
);
System.out.println("initial values = lon = 6.140263080596924 "
);
Point pixels =
CoordinateConverter.latLngToAbsolutePixelCoordsAtZoom
(46.20181075433888, 6.140263080596924, 16, 256);
System.out.println("POINT PIXEL X = " + pixels.getX());
System.out.println("POINT PIXEL Y = " + pixels.getY());
PointD latlon =
CoordinateConverter.pixelToLatLong(pixels.getX(),
pixels.getY(), 16);
System.out.println("latlon lon = " + latlon.getX());
System.out.println("latlon lat = " + latlon.getY());
}
My problem is that the results of both methods don't match ...
The latitude which was 46.20181075433888 initially, is outputed
46.2018147146228 by my pixelToLatLong method.
I must be doing some miscalculation somewhere but I don't know where.
Can anybody help me ?
thanks a lot !
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