本程式把鄉鎮縣市的 geojson 檔裡面的 MultiPolygon 當中面積最大的一塊抓出來， 變成一個單獨的 Polygon， 就可以上傳至 umap 了。

multi2maxpolygon.py

#!/usr/bin/python
# -*- coding: utf-8 -*-
# http://newtoypia.blogspot.tw/2015/08/admin-boundary.html
# 「臺灣各級行政區域(縣市/鄉鎮/村里)邊界座標檔， 自選解析度、 存成 csv、 畫成 svg 」

import json, argparse, sys, math, warnings

def area(coords):
    n = len(coords)
    if n < 3:
        return 0
    o = [ (coords[0][0]+coords[n/3][0]+coords[n*2/3][0]) / 3,
          (coords[0][1]+coords[n/3][1]+coords[n*2/3][1]) / 3 ]
    a = 0
    for i in range(len(coords)):
        p = [coords[i][0]-o[0], coords[i][1]-o[1]]
        q = [coords[(i+1)%n][0]-o[0], coords[(i+1)%n][1]-o[1]]
        a += p[0]*q[1] - p[1]*q[0]
    c = math.cos(o[1]/180*math.pi)
    # warnings.warn('# cos({0:8f} deg) == {1:8f}'.format(o[1], c))
    return abs(a*c)

def largest_piece(coords):
    if isinstance(coords[0], list):
        if isinstance(coords[0][0], (int, long, float)) and len(coords[0])==2:
            return { 'area': area(coords), 'coords': coords }
        else:
            pieces = map(largest_piece, coords)
            m = pieces[0]
            for p in pieces:
                if p['area'] > m['area']:
                    m = p
            return m
    else:
        assert False, "input data dose not look like geometry coordinates"

def multi2maxpolygon(gjdata):
    if isinstance(gjdata, dict):
        if gjdata['type'] == 'FeatureCollection':
	    gjdata['features'] = map(multi2maxpolygon, gjdata['features'])
        elif gjdata['type'] != 'Feature':
            warnings.warn('ignoring something with type=="' + gjdata['type'] + '"')
        elif not ('geometry' in gjdata):
            warnings.warn('ignoring a feature without geometry: ' + json.dumps(gjdata,indent=2,ensure_ascii=False).encode('utf8'))
        elif gjdata['geometry'] is None:
            warnings.warn('ignoring a feature with null geometry: ' + json.dumps(gjdata,indent=2,ensure_ascii=False).encode('utf8'))
        else:
            if len(args.name_field) > 0:
		gjdata['properties']['name'] = gjdata['properties'][args.name_field]
            if gjdata['geometry']['type'] == 'MultiPolygon':
                gjdata['geometry']['coordinates'] = [largest_piece(gjdata['geometry']['coordinates'])['coords']]
                gjdata['geometry']['type'] = 'Polygon'
        return gjdata
    elif isinstance(gjdata, list):
        return map(multi2maxpolygon, gjdata)

parser = argparse.ArgumentParser(description='pick the largest piece of a multipolygon in geojson files',formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('-n', '--name_field', type=str,
    default='', help='which subfield of "properties" to use as "name"?')
parser.add_argument('gjf', nargs='*', default='-', help='path to geojson files')
args = parser.parse_args()

for fn in args.gjf:
    gjfile = sys.stdin if fn == '-' else open(fn)
    gjdata = json.load(gjfile)
    print json.dumps(multi2maxpolygon(gjdata),indent=2,ensure_ascii=False).encode('utf8')
    # https://stackoverflow.com/questions/18337407/saving-utf-8-texts-in-json-dumps-as-utf8-not-as-u-escape-sequence
    if gjfile is not sys.stdin:
        gjfile.close()