[GIS] How to filter relation members in Overpass

overpass-api

My final goal is to get country borders from OSM via Overpass API.
I know my country's relation object id – http://www.openstreetmap.org/relation/59065 – and can get its details with the following query:

[out:json];
  rel(59065);
out body;

The relation members list contains ways (which comprise the country border) as well as some other nodes and relations.
How can I filter relation members to leave only way elements?

Best Answer

You cannot reduce the output of the relation itself (it will always include all elements). However, you can specify to only return ways, which are included in a relation:

[out:json];
rel(59065);
way(r);
out geom;

Try it in overpass turbo: http://overpass-turbo.eu/s/c0l and press "Run"

Related Solutions

[GIS] How to get the nodes of an area in Overpass API in the “right” order

(I edited my answer for your follow-up questions)

You have to use the order in which the node IDs are referenced by the corresponding <way> element in the XML file. In contrast, the order in which the <nodes> elements appear in the XML file is completely irrelevant.

Furthermore, as you already discovered, a relation can contain multiple separate ways forming a single polygon. Here, the relation doesn't necessarily has to reference the ways in the correct order. Consequently the correct order of the ways has to be determined by looking at the first and last node of the way. Consecutive ways will share the same last and first node.

For a neat sorting algorithm you can take a look at the source code of JOSM.

Alternatively you might take a look at overpass turbo which provides a GeoJSON output which Leaflet should be able to handle.

Getting OSM features by ID at a version number

While it's possible to fetch all relation versions along with their respective nodes and ways in a query similar to your example, Overpass turbo doesn't support exporting multiple object versions in GeoJSON.

To use this data in downstream processing, you would have to split the result produced by the following query after each "relation" element:

timeline(rel, 13249829);
foreach(
  retro(u(t["created"]))
  (
    (rel(13249829);>>;);
    out meta;
    
  );
);

The XML result adheres to the following structure:

 ... nodes and ways versions used by relation version 1...
 ... ways versions used by relation version 1...
 ... relation 1 ...

    <--- ( split result by version ) --->

 ... nodes and ways versions used by relation version 2...
 ... ways versions used by relation version 2...
 ... relation 2 ...

 (and so forth)

In a post processing step, you would need to split the result as indicated above.

Best Answer

Related Solutions

[GIS] How to get the nodes of an area in Overpass API in the “right” order

Getting OSM features by ID at a version number

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