Create a geometry from an OpenStreetMap element

geometry_from_osm.py

import requests
from functools import partial
from collections import OrderedDict
from xml.dom.minidom import parseString as xml_parse
from shapely.geometry import MultiPolygon, Polygon, Point


def osm_getter(element_id, endpoint):
    assert endpoint in ('node', 'way', 'relation')
    OSM_API_URL = 'http://www.openstreetmap.org/api/0.6/{}/{}'
    r = requests.get(OSM_API_URL.format(endpoint, element_id))
    if r.status_code == 200:
        return xml_parse(r.text)
    elif r.status_code == 404:
        raise ValueError('No such {}: {}.'.format(endpoint, element_id))
    else:
        raise ConnectionError('OSM API responded with {}.'.format(r.status_code))


get_node = partial(osm_getter, endpoint='node')
get_way = partial(osm_getter, endpoint='way')
get_relation = partial(osm_getter, endpoint='relation')


def node_to_point(node_id):
    node = get_node(node_id).getElementsByTagName('node')[0]
    return Point(map(float,
                     (node.attributes['lon'].value,
                      node.attributes['lat'].value)))


def way_to_polygon(way_id):
    points = OrderedDict()
    way_tree = get_way(way_id)
    for nd in way_tree.getElementsByTagName('nd'):
        node_id = nd.attributes['ref'].value
        points[node_id] = node_to_point(node_id)
    return Polygon(p.coords[0] for _, p in points.items())


def relation_to_multipolygon(relation_id):
    polygons = OrderedDict()
    relation_tree = get_relation(relation_id)
    for member in relation_tree.getElementsByTagName('member'):
        if member.attributes['type'].value == 'way':
            way_id = member.attributes['ref'].value
            polygons[way_id] = way_to_polygon(way_id)
    return MultiPolygon([p for _, p in polygons.items()])

Is Strawberry Fields in Central Park?

>>> way_to_polygon(161202890).contains(node_to_point(3248816675))
True

Is Vienna in Australia?

>>> relation_to_multipolygon(80500).contains(node_to_point(17328659))
# sadly this doesn't work yet

NB: relation_to_multipolygon can take a long time to run, especially for relations with many members, because API calls are not made in parallel.

It's probably better to use this database.