Gary Ruben wrote:
> I'm plotting a coverage map of a sphere using the Mollweide plot in
> basemap. The attachment is an example that is produced by sending an
> array of polygons (one polygon per row described as four corners, one
> per column) described using polar (theta) and azimuthal (phi) angles to
> the following function. As a kludge, I discard any polygons that cross
> the map boundary, but this produces artefacts and it would be better to
> subdivide these and keep the parts. I was wondering whether there's a
> function I missed that allows me to add polygons and performs the split
> across the map boundary.
>
> Gary R.
Gary: You might be able to use the _geoslib module to compute the
intersections of those polygons with the map boundary. I do a similar
thing with the coastline polygons in the _readboundarydata function.
The _boundarypolyll and _boundarypolyxy instance variables have the
vertices of the map projection region polygons in lat/lon and projection
coords. You could do somethig like this:
from mpl_toolkits.basemap import _geoslib
poly = _geoslib.Polygon(b) # a geos Polygon instance
describing your polygon)
b = self._boundarypolyxy.boundary
bx = b[:,0]; by= b[:,1]
boundarypoly = _geoslib.Polygon(b) # a geos Polygon instance
describing the map region
if poly.intersects(boundarypoly):
geoms = poly.intersection(boundarypoly)
polygons = [] # polygon intersections to plot.
for psub in geoms:
b = psub.boundary # boundary of an intersection
polygons.append(zip(b[:,0],b[:,1]))
-Jeff
>
> def Mollweide(theta, phi):
> def combinations(iterable, r):
> ''' Python 2.6 itertools function'''
> # combinations('ABCD', 2) --> AB AC AD BC BD CD
> # combinations(range(4), 3) --> 012 013 023 123
> pool = tuple(iterable)
> n = len(pool)
> if r > n:
> return
> indices = range(r)
> yield tuple(pool[i] for i in indices)
> while True:
> for i in reversed(range(r)):
> if indices[i] != i + n - r:
> break
> else:
> return
> indices[i] += 1
> for j in range(i+1, r):
> indices[j] = indices[j-1] + 1
> yield tuple(pool[i] for i in indices)
>
>
> def boundary_crossed(pts):
> crossed = False
> for c in combinations(pts, 2):
> if abs(c[0]-c[1])>180:
> crossed = True
> break
> return crossed
>
> # Make Mollweide plot
> m = Basemap(projection='moll', lon_0=0, resolution='c')
>
> # draw the edge of the map projection region (the projection limb)
> m.drawmapboundary()
> # draw lat/lon grid lines every 30 degrees.
> m.drawmeridians(np.arange(0,360,30), dashes=[10,0])
> m.drawparallels(np.arange(-90,90,30), dashes=[10,0])
>
> ax = plt.gca() # get current axes instance
> for i in range(theta.shape[0]):
> pts = np.vstack((theta[i], phi[i])).T
> if boundary_crossed(pts[:,1]):
> continue # skip polys that cross the map boundary
>
> polypts = [m(pt[1], pt[0]) for pt in pts]
> poly = Polygon(polypts, facecolor="b", edgecolor="None",
> alpha=0.5)
> ax.add_patch(poly)
>
> ------------------------------------------------------------------------
>
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