Computes the center of a shapefile multi-polygon

shape_center.py

"""
computes the center of a multi-polygon
"""
def shape_center(shape):
	"""
	computes the center of gravity of a shapefile multi-polygon
	shape must be an instance of shapefile._Shape of the Python Shapefile Library
	http://packages.python.org/Python%20Shapefile%20Library/
	Polygon class comes from here
	http://pypi.python.org/pypi/Polygon/1.17
	"""
	from Polygon import Polygon
	parts = shape.parts[:]
	parts.append(len(shape.points))
	
	# the center computation produces false results for 
	# countries whose shapes cross the 180° longitude
	# we need to check this
	far_east = False
	far_west = False
	for i in range(len(parts)-1):
		pts = shape.points[parts[i]:parts[i+1]]
		if len(pts) == 0: continue
		if pts[0][0] < -90:
			far_west = True
		if pts[0][0] > 90:
			far_east = True
	
	# now we convert the Shape into a Polygon
	poly = Polygon()
	for i in range(len(parts)-1):
		pts = shape.points[parts[i]:parts[i+1]]
		if far_east and far_west:
			# correct points if country crosses 180° lon
			for j in range(len(pts)):
				if pts[j][0] < 0: pts[j][0] += 360
		poly.addContour(pts)
	# and return its center of gravity
	return poly.center()