[Talk-GB] Administrative boundaries: polygons or polylines?

Colin Smale colin.smale at xs4all.nl
Sun Jan 17 18:33:59 UTC 2016


Hi Bob, 

They all seem to be made up of multiple ways. I noticed that Bix and
Assendon has role=outer consistently applied, whereas the others have no
explicit role on the ways. This is implicitly equivalent to role=outer
but having explicit roles for the members in a relation may be
considered "best practice". 

I don't know which software you are using which identifies them as
"polygon" or "polyline", but is it possibly related to the order of the
ways in the relation? I am thinking that if the ways are listed in the
"right" order and can be linked together head-to-tail without
re-ordering the ways or reversing the nodes, it might be identified as a
simple polygon, whereas if the ways need re-ordering or reversing to
make a complete ring then the software may just say "it's a list of
lines i.e. a polyline". 

I will take a closer look at the relations you mentioned to see if that
may be the case, but I assume you realise that OSM doesn't "do" polygons
as a native data type, and allows the members of a multipolygon (and by
implication, boundary) relation to be in any order... When lines are
shared between boundaries of adjacent areas, it is not topologically
possible to have them in the perfect order in all the relations at the
same time.

Best regards, 

Colin 

On 2016-01-17 19:12, Bob Hawkins wrote:

> Colin 
> I thank you for your prompt reply. 
> I have been investigating further in the meantime to try to establish what makes a difference.  I wondered if it lay with "type=boundary" or its absence but my relations I have checked all have type=boundary.  I wonder if you have time to look at South Oxfordshire around Henley-on-Thames, say.  Bix and Assendon civil parish shows as a polygon; adjacent Nettlebed, Highmoor and Rotherfield Greys do not, yet have type=boundary in their relations.  Your experience with administrative boundaries in OSM might make it easier to spot the issue. 
> 
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