diff options
Diffstat (limited to 'webapp/django/contrib/gis/maps/google/zoom.py')
| -rw-r--r-- | webapp/django/contrib/gis/maps/google/zoom.py | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/webapp/django/contrib/gis/maps/google/zoom.py b/webapp/django/contrib/gis/maps/google/zoom.py new file mode 100644 index 0000000000..abc3fbf9d5 --- /dev/null +++ b/webapp/django/contrib/gis/maps/google/zoom.py @@ -0,0 +1,161 @@ +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 |