Converting OSM relation to a list of lat/lng points?

Question

I would like a convert a relation (e.g. https://openstreetmap.org/relation/2851613) to a list of coordinates that can be displayed on a map, potentially using geojson. I ported over this converter https://wiki.openstreetmap.org/wiki/Relations/Relations_to_GPX (the converted code is at https://github.com/e-mission/e-mission-eval-public-data/blob/master/spec_creation/autofill_eval_spec.py#L71) and it works iff the relations are sane.

However, many relations are malformed. Although the relation spec https://wiki.openstreetmap.org/wiki/Relation says that the members have to be ordered, this relation for the route of bus #52 (https://www.openstreetmap.org/relation/8094017) has the following way order

Way West El Camino Real (516422275) as forward
Way Castro Street (417101895) as forward
Way North El Monte Avenue (52869637) as forward 

which is clearly incorrect since since the 52 goes from Castro to El Monte via El Camino. So if I use the algorithm above, I end up with a map like so

At the same time, OSM displays the route without any zigzags, so it is clearly able to order the relation members correctly even if they are not ordered originally.

In response to @neogeomat, I looked at the nodes displayed by overpass and the nodes displayed by the reconstruction algorithm above for a correctly created relation, and it looks like overpass returns fewer nodes than the algorithm above.

Does anybody know how OSM displays relations, or have a pointer to an algorithm that deals gracefully with malformed relations?

Answer 1

You can try osmtogeojson (used by overpass turbo) or one of the alternatives listed at the GeoJSON OSM wiki page. Unfortunately I have no experience with any of them and I'm unsure which one supports sorting of relation members.

Answer 2

Your underlying assumption that the members of a (route-)relation are ordered is not correct. The quote in the API doc simply means that the order in which you provided the member elements will be maintained.

This should not be a surprise as the API does not and cannot know the semantics (for example in which way it should be ordered) of a specific relation type.

A superficial check of the code of osmtogeojson would indicate that it indeed does properly create geometries for route relations (and for multipolygons) by connecting adjacent ways.

Note there are some cases in which automatic ordering will not produce the correct results, but as long as you are only rendering and not traversing the ways that doesn't really matter.

Answer 3

OSM does not display relations. relations are not geographical objects and cannot be dispalyed as is, so only the members are displayed. to convert relation to nodes and use you can use overpass-api. e.g. here and here

Answer 4

I tested your example (https://overpass-turbo.eu/s/UA9) with osm2geojson. And result looks good.

Example code:

import requests
import urllib
import codecs
import json
from osm2geojson import json2geojson

OVERPASS = "https://overpass-api.de/api/interpreter/"


def overpass_call(query):
    encoded = urllib.parse.quote(query.encode('utf-8'), safe='~()*!.\'')
    r = requests.post(OVERPASS,
                      data=f"data={encoded}",
                      headers={'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8'})
    if not r.status_code is 200:
        raise requests.exceptions.HTTPError('Overpass server respond with status '+str(r.status_code))
    return r.text


def save_data(data, geom_file):
    json_data = json.dumps(data, indent=2)
    f = codecs.open(geom_file, 'w')
    f.write(json_data)
    f.close()


data = overpass_call(f"""
    [out:json];
    rel(8094017);
    out geom;
""")

geojson_data = json2geojson(data)
save_data(geojson_data, 'road.geojson')