I am new to Shapely (but enthusiastic about it), and recently I've discovered a bit of a road bump.
I have a polygon shapefile that I am reading in via Fiona. This shapefile contains BOTH polygon and multipolygon items and I need to build an array for each feature of all the coordinates within it (i.e. both exterior and/or interior). Notably, two of the polygon items have interior rings (and they are valid).
I seem to have no problem accessing the exterior coordinates of the polygon(s)/multipolygon(s) ... but I am not pulling anything for the interior coordinates.
Do I need to take a new approach here (i.e. LinearRings)...?
def convert_polygons(inFile):
for polys in fiona.open(inFile):
myShape = shape(polys['geometry'])
exterior_poly = 0
interior_poly = 0
if isinstance(myShape, Polygon):
print "yes, I am a polygon"
# count how many points for each interior polygon
try:
interior_poly += len(myShape.interior.coords)
except:
pass
# count how many points for each exterior polygon
exterior_poly += len(myShape.exterior.coords)
geomArray = asarray(myShape.exterior)
print geomArray
print "number of interior points in polygon " + str(interior_poly)
print "number of exterior points in polygon " + str(exterior_poly)
elif isinstance(myShape, MultiPolygon):
print "yes, I am a MultiPolygon"
# count how many points for each interior polygon
try:
interior_poly += len(myShape.interior.coords)
except:
pass
try:
# count how many points for each exterior polygon
exterior_poly += len(myShape.exterior.coords)
except:
pass
try:
geomArray = asarray(myShape.interior)
except:
pass
try:
geomArray = asarray(myShape.exterior)
except:
pass
print geomArray
print "number of interior points in polygon " + str(interior_poly)
print "number of exterior points in polygon " + str(exterior_poly)
Interior and exterior rings are structured differently. For any polygon, there is always 1 exterior ring with zero or more interior rings.
So looking at the structure of a geometry, exterior
is a LinearRing
object, and interiors
is a list of zero or more LinearRing
objects. Any LinearRing
object will have coords
, which you can slice to see a list of the coordinates with coords[:]
.
The following is a function that returns a dict of lists of exterior and interior coordinates:
def extract_poly_coords(geom):
if geom.type == 'Polygon':
exterior_coords = geom.exterior.coords[:]
interior_coords = []
for interior in geom.interiors:
interior_coords += interior.coords[:]
elif geom.type == 'MultiPolygon':
exterior_coords = []
interior_coords = []
for part in geom:
epc = extract_poly_coords(part) # Recursive call
exterior_coords += epc['exterior_coords']
interior_coords += epc['interior_coords']
else:
raise ValueError('Unhandled geometry type: ' + repr(geom.type))
return {'exterior_coords': exterior_coords,
'interior_coords': interior_coords}
E.g.:
extract_poly_coords(myShape)