Convert decimal geoloxatilation to cardinal point (x,y)

[joaomello]

I did in C# in this way:
public static PointF ConverterCoordenadaParaCartesiana(double latitude, double longitude)
{
double latitude_ = (Math.PI / 2) + (latitude * Math.PI / 180.0);
double longitude_ = (longitude * Math.PI / 180.0);

double x = 6378100 * Math.Cos(longitude_) * Math.Sin(latitude_);
double y = 6378100 * Math.Sin(longitude_) * Math.Sin(latitude_);
double z = 6378100 * Math.Cos(latitude_);

return new PointF(Convert.ToSingle(x), Convert.ToSingle(y));
}

but it's very diferent when I compare to google something like a "group of way points", has a better way?
Sorry about my english..

Mello!