/* Scale from pixels to radians in the projection plane
 * x0,y0   Reference pixel index
 * dx,dy   Degrees per pixel
 */
function scaler(x0, y0, dx, dy) {

    dx = dx*Math.PI/180
    dy = dy*Math.PI/180    
    this.rscale = scale;

    /** Scale pixels to radians
      * x,y  pixel location.
      */
    function scale(x, y) {
        return {x:(x-x0)*dx, y:(y-y0)*dy}
    }
}