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