Converting long/lat to pixel x/y, given a zoom-level

Question

I'm trying to develop a page in ASP.NET that will act as a tile-server for a Google Map

It will pull a collection of latitude/longitude points from the database, then render them as small red dots on a transparent background, given a zoom-level (default: 15).

It will then return the result as a GIF-type image.

Have any algorithms or libraries been developed that allow me to take this set of latitudes/longitudes and convert them to a set of 2D pixel co-ordinates, given a zoom level?

(This is all being done server-side, so I can't use the Google Maps API.)

Update: Found a code-sample in Perl that does something similar:

http://blog.barros.ws/2009/03/06/convert-lat-lng-and-zoom-values-to-pixel-xy-on-a-map/

Trouble is, I don't know Perl, and don't really have the time to crack open a book and learn it.

Can anyone help me to decipher what's going on in this function?

sub Google_Coord_to_Pix
{
    my $value   = shift ;
    my $lat = shift ;
    my $lng = shift ;
    my @d       = ( ) ; 
    my $e       = 0 ;

    $d[1] = sprintf("%0.0f", $$value{'bmO'} + $lng * $$value{'pixLngDeg'} ) ;

    $e = sin($lat * $$value{'Wa'}) ;

    if( $e > 0.99999 )
    {
        $e = 0.99999 ;
    }

    if( $e < -0.99999 )
    {
        $e = -0.99999 ;
    }

    $d[0] = sprintf("%0.0f", $$value{'bmO'} + 0.5 * log((1 + $e) / (1 - $e)) * (-1) * $$value{'pixLngRad'} ) ;

    return (@d) ;
}

Chris B Chris B · Accepted Answer · 2009-10-21T18:12:23

Here's some code I'm currently using. It's in PHP.

// Returns longitude in pixels at a certain zoom level
function lonToX($lon, $zoom) {
    $offset = 256 << ($zoom-1);
    return round($offset + ($offset * $lon / 180));
}
// Returns latitude in pixels at a certain zoom level
function latToY($lat, $zoom) {
    $offset = 256 << ($zoom-1);
    return round($offset - $offset/pi() * log((1 + sin($lat * pi() / 180)) / (1 - sin($lat * pi() / 180))) / 2);
}

Based on code from this page, written by this guy.

Good luck!

Update: This map is a great way to help understand how tiles work in Google Maps

Edit: Here's an equivalent set of functions in VB.NET:

Public Function LonToX(Lon As Double, Zoom as UInteger) As UInteger
    Dim Offset = 256 << (Zoom - 1)
    Return Math.Round(Offset + (Offset * Lon / 180))
End Function

Public Function LatToY(Lat As Double, Zoom as UInteger) As UInteger
    Dim Offset = 256 << (Zoom - 1)
    Return Math.Round(Offset - Offset / Math.Pi * Math.Log((1 + Math.Sin(Lat * Math.Pi / 180)) / (1 - Math.Sin(Lat * Math.Pi / 180))) / 2)
End Function

And in C#:

public uint lonToX(double lon, uint zoom) {
    uint offset = 256 << (zoom - 1);
    return Math.Round(offset + (offset * lon / 180));
}

public uint latToY(double lat, uint zoom) {
    uint offset = 256 << (zoom - 1);
    return Math.Round(offset - offset / Math.Pi * Math.Log((1 + Math.Sin(lat * Math.Pi / 180)) / (1 - Math.Sin(lat * Math.Pi / 180))) / 2);
}

Converting long/lat to pixel x/y, given a zoom-level

2 Answers