text: from django.contrib.gis.geos import GEOSGeometry, LinearRing, Polygon, Point
  from django.contrib.gis.maps.google.gmap import GoogleMapException
  from math import pi, sin, cos, log, exp, atan
  
  # Constants used for degree to radian conversion, and vice-versa.
  DTOR = pi / 180.
  RTOD = 180. / pi
  
  class GoogleZoom(object):
      """
      GoogleZoom is a utility for performing operations related to the zoom
      levels on Google Maps.
  
      This class is inspired by the OpenStreetMap Mapnik tile generation routine
      `generate_tiles.py`, and the article "How Big Is the World" (Hack #16) in
      "Google Maps Hacks" by Rich Gibson and Schuyler Erle.
  
      `generate_tiles.py` may be found at:
        http://trac.openstreetmap.org/browser/applications/rendering/mapnik/generate_tiles.py
  
      "Google Maps Hacks" may be found at http://safari.oreilly.com/0596101619
      """
      
      def __init__(self, num_zoom=19, tilesize=256):
          "Initializes the Google Zoom object."
          # Google's tilesize is 256x256, square tiles are assumed.
          self._tilesize = tilesize
          
          # The number of zoom levels
          self._nzoom = num_zoom
  
          # Initializing arrays to hold the parameters for each one of the 
          # zoom levels.
          self._degpp = [] # Degrees per pixel
          self._radpp = [] # Radians per pixel
          self._npix  = [] # 1/2 the number of pixels for a tile at the given zoom level
          
          # Incrementing through the zoom levels and populating the parameter arrays.
          z = tilesize # The number of pixels per zoom level.
          for i in xrange(num_zoom):
              # Getting the degrees and radians per pixel, and the 1/2 the number of
              # for every zoom level.
              self._degpp.append(z / 360.) # degrees per pixel
              self._radpp.append(z / (2 * pi)) # radians per pixl
              self._npix.append(z / 2) # number of pixels to center of tile
  
              # Multiplying `z` by 2 for the next iteration.
              z *= 2
  
      def __len__(self):
          "Returns the number of zoom levels."
          return self._nzoom
  
      def get_lon_lat(self, lonlat):
          "Unpacks longitude, latitude from GEOS Points and 2-tuples."
          if isinstance(lonlat, Point):
              lon, lat = lonlat.coords
          else:
              lon, lat = lonlat
          return lon, lat
  
      def lonlat_to_pixel(self, lonlat, zoom):
          "Converts a longitude, latitude coordinate pair for the given zoom level."
          # Setting up, unpacking the longitude, latitude values and getting the
          # number of pixels for the given zoom level.
          lon, lat = self.get_lon_lat(lonlat)
          npix = self._npix[zoom]
  
          # Calculating the pixel x coordinate by multiplying the longitude value
          # with with the number of degrees/pixel at the given zoom level.
          px_x = round(npix + (lon * self._degpp[zoom]))
  
          # Creating the factor, and ensuring that 1 or -1 is not passed in as the 
          # base to the logarithm.  Here's why:
          #  if fac = -1, we'll get log(0) which is undefined; 
          #  if fac =  1, our logarithm base will be divided by 0, also undefined.
          fac = min(max(sin(DTOR * lat), -0.9999), 0.9999)
  
          # Calculating the pixel y coordinate.
          px_y = round(npix + (0.5 * log((1 + fac)/(1 - fac)) * (-1.0 * self._radpp[zoom])))
  
          # Returning the pixel x, y to the caller of the function.
          return (px_x, px_y)
  
      def pixel_to_lonlat(self, px, zoom):
          "Converts a pixel to a longitude, latitude pair at the given zoom level."
          if len(px) != 2:
              raise TypeError('Pixel should be a sequence of two elements.')
  
          # Getting the number of pixels for the given zoom level.
          npix = self._npix[zoom]
  
          # Calculating the longitude value, using the degrees per pixel.
          lon = (px[0] - npix) / self._degpp[zoom]
  
          # Calculating the latitude value.
          lat = RTOD * ( 2 * atan(exp((px[1] - npix)/ (-1.0 * self._radpp[zoom]))) - 0.5 * pi)
  
          # Returning the longitude, latitude coordinate pair.
          return (lon, lat)
      
      def tile(self, lonlat, zoom):
          """
          Returns a Polygon  corresponding to the region represented by a fictional
          Google Tile for the given longitude/latitude pair and zoom level. This
          tile is used to determine the size of a tile at the given point.
          """
          # The given lonlat is the center of the tile.
          delta = self._tilesize / 2
  
          # Getting the pixel coordinates corresponding to the
          # the longitude/latitude.
          px = self.lonlat_to_pixel(lonlat, zoom)
  
          # Getting the lower-left and upper-right lat/lon coordinates
          # for the bounding box of the tile.
          ll = self.pixel_to_lonlat((px[0]-delta, px[1]-delta), zoom)
          ur = self.pixel_to_lonlat((px[0]+delta, px[1]+delta), zoom)
  
          # Constructing the Polygon, representing the tile and returning.
          return Polygon(LinearRing(ll, (ll[0], ur[1]), ur, (ur[0], ll[1]), ll), srid=4326)
          
      def get_zoom(self, geom):
          "Returns the optimal Zoom level for the given geometry."
          # Checking the input type.
          if not isinstance(geom, GEOSGeometry) or geom.srid != 4326:
              raise TypeError('get_zoom() expects a GEOS Geometry with an SRID of 4326.')
  
          # Getting the envelope for the geometry, and its associated width, height
          # and centroid.
          env = geom.envelope
          env_w, env_h = self.get_width_height(env.extent)
          center = env.centroid
  
          for z in xrange(self._nzoom):
              # Getting the tile at the zoom level.
              tile_w, tile_h = self.get_width_height(self.tile(center, z).extent)
  
              # When we span more than one tile, this is an approximately good
              # zoom level.
              if (env_w > tile_w) or (env_h > tile_h):
                  if z == 0: 
                      raise GoogleMapException('Geometry width and height should not exceed that of the Earth.')
                  return z-1
          
          # Otherwise, we've zoomed in to the max.
          return self._nzoom-1
  
      def get_width_height(self, extent):
          """
          Returns the width and height for the given extent.
          """
          # Getting the lower-left, upper-left, and upper-right
          # coordinates from the extent.
          ll = Point(extent[:2])
          ul = Point(extent[0], extent[3])
          ur = Point(extent[2:])
          # Calculating the width and height.
          height = ll.distance(ul)
          width  = ul.distance(ur)
          return width, height
  