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
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