Differentiate Inner vs Outer multipolygon from Overpass API

Question

I'm making an API request to OSM with a bounding box query.

When I'm getting an object back that's a multipolygon, I'm unclear how I can differentiate if the relation is inner vs outer. I figured that would be included in the API response, but it's not. What am I missing? Is there some way I'm able to differentiate between the two?

{
          "type" => "Feature",
            "id" => "relation/296087",
    "properties" => {
               "timestamp" => "2018-07-14T19:04:53Z",
                 "version" => "5",
               "changeset" => "60718245",
                    "user" => "escallic",
                     "uid" => "2110789",
               "addr:city" => "Isla Vista",
             "addr:county" => "Santa Barbara",
        "addr:housenumber" => "6585",
           "addr:postcode" => "93117",
             "addr:street" => "El Colegio Road",
                "building" => "apartments",
         "building:levels" => "2",
                    "name" => "Tropicana Gardens",
                    "type" => "multipolygon",
                      "id" => "relation/296087"
    },
      "geometry" => {
               "type" => "Polygon",
        "coordinates" => [ ..... ]
      }

}

Update The query I'm using to make the request is below: http://overpass-api.de/api/map?bbox=-119.86475115542854%2C34.410451138127065%2C-119.85719244457147%2C34.416207261872934

I'm then taking the data (XML) and using the npm package OSMToGeoJSON to convert it into JSON. I believe that is where the relation information is being lost, as I see it in the raw XML.

How would one make a query for all data in a bounding box, but request JSON. I tried adding the query param data=[out:json]; to my response, but I did not get any data back>

I need the inner/outer data for the polygons, because I am re-drawing these maps. I work on a project where were turn these maps into tactile maps for the blind community. If I don't know whether the polygon is supposed to be inner vs outer, it ends up rendering incorrectly.

Answer 1

There is no such thing as an "inner" or "outer" polygon. There are "inner" or "outer" rings of a polygon, and in OSM these rings can consist of one or more ways. The individual members of a multipolygon relation in OSM can have "inner" or "outer" roles hinting at whether they are part of an inner or outer ring, but these roles are neither required nor guaranteed to be there. Therefore, any algorithm that turns OSM data into geometries - like the "OsmToGeoJSON" you claim to be using - needs to apply logic to build correct polygons.

Once a GeoJSON has been built, there's nothing you can or have to do - either the GeoJSON contains the correct polygons (with potential holes in them) or it doesn't. At this point, any inner/outer information is not relevant to you any more, it has been processed into the polygon geometry.

If OsmToGeoJSON does not produce correct polygons for you then this will be fore one of two reasons:

(1) it is implemented sloppily or incompletely (a working algorithm for polygon construction is described in https://wiki.openstreetmap.org/wiki/Relation:multipolygon/Algorithm) or

(2) it cannot build some polygons correctly because the response you get from the "map" request lacks referential integrity, i.e. some relations reference ways that are not contained in the response.

Answer 2

2

GeoJSON works differently than the OSM data model: It doesn't represent polygons-with-holes as relations with "inner" and "outer" members. In fact, the OSM concept of a "relation" doesn't really exist in GeoJSON.

Instead, the "coordinates" member of a GeoJSON Polygon will simply consist of multiple arrays of coordinates, the first of which represents the outer ring of the polygon, and the remaining of which represent the inner rings. See the relevant section in the GeoJSON spec.

So if OsmToGeoJSON is working correctly, the part that you omitted here

"coordinates" => [ ..... ]

should contain the coordinates of the inner rings and outer ring of your polygon.

permanent link

answered 21 Dec '18, 11:13

Tordanik
12.0k●15●106●147
accept rate: 35%

Wow thank you. Yes I've seen the multiple sets of coordinates/arrays. The first array is consistently the outer ring?

This would be a super helpful assumption to make when drawing polygons, as I was currently going the route of using/writing a function that detects if a polygon is within any of the other polygons we receive, which if it's not needed, would make things much easier.

(22 Dec '18, 17:17) kida001

@kida001: The first array should consistently be the outer ring according to the GeoJSON specification. I haven't checked myself if OsmToGeoJSON does implement this correctly, but I would start from the assumption that it does.

(22 Dec '18, 19:22) Tordanik

Differentiate Inner vs Outer multipolygon from Overpass API

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