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I am currently struggling with the same problem. I am working on a robotics control system where one or more robots progress are tracked and displayed on a map. The OpenStreetMap tile system is perfect for this. I can successfully download the tiles stitch them together, scroll, zoom etc.

I am however having difficulty finding the pixel in a tile which represents a specific lat/lon.

I currently have this which I have ported from tangoGPS (www.tangogps.org).

    public double ConvertDegreesToRadians(double degrees)
    {
        double radians = (Math.PI / 180) * degrees;
        return (radians);
    }

    public static double atanh(double x)
    {
        if (x > 1.0 || x < -1.0) 
            throw new ArithmeticException("range exception");

        return 0.5 * Math.Log((1.0 + x) / (1.0 - x));
    }

    public int LatToPixel(int ZoomLevel, double Lat)
    {
        double latm = atanh(Math.Sin(lat)); 
        return (int)Math.Truncate(-((latm * 256 * Math.Exp(ZoomLevel * Math.Log(2))) / (2 * Math.PI)) + 
                                   (Math.Exp(ZoomLevel * Math.Log(2)) * (256 / 2))); 
    }

    public int LonToPixel(int ZoomLevel, double Lon)
    {
        return (int)Math.Truncate(((Lon * 256 * Math.Exp(ZoomLevel * Math.Log(2))) / (2 * Math.PI)) + 
                                  (Math.Exp(ZoomLevel * Math.Log(2)) * (256 / 2))); 
    }

int xpixel = LonToPixel(zoomlevel, ConvertDegreesToRadians(poilon));
int ypixel = LatToPixel(zoomlevel, ConvertDegreesToRadians(poilat));

The Lon values seems ok, however the Lat does not. Did I make a mistake in the porting? Does anyone know of a diffirent source I can look at or some documentation on the formulas?

Adi