| 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 |