[GeoJSON] polygon orientation, trying number 2

[Jukka Rahkonen]

Hi Sean,

Would you mind to check also if what is written about the right-hand 
rule is correct:


"Polygon rings MUST follow the right-hand rule for orientation
       (counter-clockwise external rings, clockwise internal rings)"

At least the ESRI shapefile specification defines the right-hand rule in 
an opposite way:

"The order of vertices or orientation for a ring indicates which side of 
the ring
is the interior of the polygon. The neighborhood to the right of an 
observer walking along
the ring in vertex order is the neighborhood inside the polygon. 
Vertices of rings defining
holes in polygons are in a counterclockwise direction. Vertices for a 
single, ringed
polygon are, therefore, always in clockwise order."

For me it seems that GeoJSON specification is following the left-hand 
rule, not the right-hand rule.

-Jukka Rahkonen-


Sean Gillies kirjoitti 2016-07-16 03:38:
> Hi Anna,
> 
> Thanks for letting us know about this! I've updated the examples in
> our GitHub repo and sent a patch along to the RFC Editor. When the
> GeoJSON standard is published, the examples will have the correct
> orientation: exterior rings that wind counter-clockwise and interior
> rings that wind clockwise.
> 
> See https://github.com/geojson/draft-geojson/issues/231 for the
> specific changes.
> 
> Yours,
> 
> On Thu, Jul 14, 2016 at 4:56 AM, Anna Eivazi
> <Anna.Eivazi at sharpershape.com> wrote:
> 
>> Hello geojson-gurus,
>> 
>> First of all, thank you for all your good work about geojson, we
>> love to use it!;)
>> 
>> I’ve got a question after trying to figure out which orientation
>> to use for polygons.
>> 
>> So I’ve read your latest specs:
>> 
>> https://datatracker.ietf.org/doc/draft-ietf-geojson/?include_text=1
>> 
>> Here it is said:
>> 
>> Polygon rings MUST follow the right-hand rule for orientation
>> 
>> (counter-clockwise external rings, clockwise internal rings).
>> 
>> (clear answer to my question, right?)
>> 
>> But a bit upper in the same doc Polygon section:
>> 
>> A.3.  Polygons
>> 
>> Coordinates of a Polygon are an array of LinearRing (see
>> 
>> Section 3.1.6) coordinate arrays.  The first element in the
>> array
>> 
>> represents the exterior ring.  Any subsequent elements represent
>> 
>> 
>> interior rings (or holes).
>> 
>> No holes:
>> 
>> {
>> 
>> "type": "Polygon",
>> 
>> "coordinates": [
>> 
>> [
>> 
>> [100.0, 0.0],
>> 
>> [100.0, 1.0],
>> 
>> [101.0, 1.0],
>> 
>> [101.0, 0.0],
>> 
>> [100.0, 0.0]
>> 
>> ]
>> 
>> ]
>> 
>> }
>> 
>> That would mean clockwise orientation…?
>> 
>> So, which one u are planning to use in future? So we will stick to
>> it too!
>> 
>> Thanks a lot!
>> 
>> Best regards,
>> 
>> Anna Eivazi
>> 
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> 
> --
> Sean Gillies
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