code: def screen_to_latlon(self, x, y):
          
          xtile = 1. * x / TILE_SIZE + self.xtile
          ytile = 1. * y / TILE_SIZE + self.ytile
          return self.num2deg(xtile, ytile, self.zoom)
docs: Return the latitude and longitude corresponding to a screen point
          :param x: screen x
          :param y: screen y
          :return: latitude and longitude at x,y