question: public long latToPixel ( double lat ) { double invCos = 1 / Math . cos ( Math . toRadians ( lat ) ) ; double tan = Math . tan ( Math . toRadians ( lat ) ) ; double ln = Math . log ( tan + invCos ) ; return ( long ) ( ( 1 - ln / Math . PI ) * Math . pow ( 2 , zoom - 1 ) * 256 ) ; }
target:  convert latitude to pixel value 