[OSM-talk-ie] Defaced townlands

Andy Townsend ajt1047 at gmail.com
Thu Feb 4 23:26:01 UTC 2021


On 04/02/2021 21:28, Colm Moore wrote:
> Hi,
>
> Recently, I've been adding Logainm details to OSM, both streets and townlands. There are about 61,000 townlands, but only 51,800 Logainm entries on OSM, although with some of those entries being other types of area and highways, that means there are a lot of unreconciled townlands.
>
>  From time to time townland and other boundaries on OSM are altered, for whatever reason. Is there anyone doing maintenance on boundaries? I can identify issues, but I don't know how to fix them.

One think that might be happening is that the things that form the edges 
of townlands might get edited (in line with imagery) and that might 
cause a problem with the area of the townland - it might not form a 
valid polygon in the database afterwards.

As an example, (one that isn't broken) is Youngfield - 
https://www.openstreetmap.org/relation/5987999 .  As part of its 
boundary it has a bit of river and a bit of wall 
https://www.openstreetmap.org/way/385238056 .  If someone extends the 
wall it might make the townland not valid as a polygon; it might end up 
as a ring with a "sticky out bit".

This sort of thing can happen to anything in OSM that is a relation 
composed of ways.  I wrote a diary entry about a similar problem with 
national parks in IE/GB - 
https://www.openstreetmap.org/user/SomeoneElse/diary/395232 .  You 
could, in theory, apply the same process to townlands, but the tricky 
bit is that there are many thousands of townlands - far more than there 
are national parks in IE and GB.

If the problem is accidentally broken geometry, "how to fix" is the same 
as in that diary entry though - load the relation in JOSM, validate, and 
investigate errors and warnings.  You may need to download extra data to 
resolve some of the warnings, but things such as gaps in multipolygons 
will be revealed by that process and can be fixed manually.

Someone could, if they has access to a rendering database loaded with 
Irish data, also do something like I've written in that diary entry for 
"Detecting future problems" (and of course someone may be doing that 
already - I've not actually noticed any geometrically broken townlands).

Best Regards,

Andy






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