gistfile1.py

    def get_tile_origin(self, tile_x, tile_y, zoom):
        '''Return the lat, lng of the northwest corner of the tile.'''
        tile_width = 360.0/pow(2,zoom) # degrees
        tile_height = tile_width/2        # degrees
        n = 90 - tile_y * tile_height
        w = -180 + tile_x * tile_width
        return (n,w)
        
    def get_bb(self, tile_x, tile_y, zoom):
        """Return bounding coordinates (n, e, s, w) of the tile."""
        tile_width = 360.0/pow(2,zoom) # degrees
        tile_height = tile_width/2   # degrees
        n = 90 - tile_y * tile_height
        s = n - tile_height
        w = -180 + tile_x * tile_width
        e = w + tile_width
        return (n, e, s, w)
        
    def world_coord_from_point(self, zoom, lat, lng):
        '''Return the number of pixels in x, and y given 0,0 at lat=90, lng=-180'''
        pixel_x = int(256*(lng+180)/360.0 * pow(2,zoom))
        pixel_y = int(256*(90-lat)/180.0 * pow(2,zoom))
        return (pixel_x, pixel_y)

    def tile_from_point(self, zoom, lat, lng):
        ''' Return a tuple containing the x, y tile index.'''
        pixel_x, pixel_y = self.world_coord_from_point(zoom, lat, lng)
        tile_x = int(pixel_x / 256.0)
        tile_y = int(pixel_y / 256.0)
        return (tile_x, tile_y)
          
    def get_tile_coordinate(self, zoom, lat, lng):
        """Returns the offset in pixels from origin of the tile in the nw corner."""
        tile_x, tile_y = self.tile_from_point(zoom, lat, lng)
        n, w = self.get_tile_origin(tile_x, tile_y, zoom)
        world_x, world_y = self.world_coord_from_point(zoom, lat, lng)
        left, top = self.world_coord_from_point(zoom,n,w)
        x_offset = world_x - left
        y_offset = world_y - top
        return (x_offset, y_offset)