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All,
We try to generate contour polygons from an unstructured triangular grid
stored in a netcdf file:
 import netCDF4
 import matplotlib.tri as tri
 # read data
 var = netCDF4.Dataset('filename.cdf').variables
 x = var['x'][:]
 y = var['y'][:]
 elems = var['element'][:,:]-1
 data = var['attrname'][:] 
 # create triangulation
 triang = tri.Triangulation(x, y, triangles=elems)
 # generate contour polygons
 levels = numpy.linspace(0, maxlevel, num=numlevels)
 contour = plt.tricontourf(triang, data, levels=levels)
 # extract geometries
 for coll in contour.collections:
 # handle all geometries for one level
 for p in coll.get_paths():
 polys = p.to_polygons()
 ...
At this point we assume, that polys[0] is a linear ring to be interpreted as
a polygon exterior and polys[1:] are the corresponding interiors for
polys[0]. 
Here are our questions: 
Is this assumption correct? 
Is there any detailed documentation describing the structure of the returned
geometries? 
Are the linear rings supposed to be correctly oriented (area > 0 for
exteriors and area < 0 for the interiors)?
Thanks!
Regards Hartmut
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