[OSM-talk-ie] Split townlands

Rory McCann rory at technomancy.org
Wed Apr 1 14:32:07 UTC 2015

Hash: SHA1

On 01/04/15 15:40, Rachel Murphy wrote:
> I have a townland that is split in two by a road (Ballysteena, Co
> Tipperary).  How do you handle this? Do you have to create two separate
> areas/polygons and then combine somehow?

Hi Rachel,

It's actually not that hard. You only create one townland relation, then add all the edges for the first part in as members, with the 'outer' role.... Then just add all the edges of the other part (again with 'outer')!

Here's an example: http://www.openstreetmap.org/relation/3341134

You can see the little island to the south ( http://www.openstreetmap.org/way/4563271 ) is just added as another member of the townland relation with an "outer" role. When you do right it in JOSM, you should see 2 "loops" in the relation window (if you have 3 parts, 3 loops etc). The OSM wiki goes into more details: http://wiki.openstreetmap.org/wiki/Relation:multipolygon#Two_disjunct_outer_rings and http://wiki.openstreetmap.org/wiki/Relation:multipolygon#Two_disjunct_outer_rings_and_multiple_ways_forming_a_ring

It's not very common for townlands (since they are so small), but you can often have civil parishes, baronies and counties made up of more than one bit (e.g. a civil parish: http://www.townlands.ie/clare/kilmacrehy/ ). County Galway, for example, is made up of the mainland, and the Aran Islands (and lots of other islands too). That is all mapped in this same way.

The way it works under the hood is that software that does things with OSM data (e.g. to make a map) will try to join up all the edges together if they touch at an end. At the end of that process there might be one polygon, or many polygons. So it all works out.

Hope that helps

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