[GeoJSON] polygon orientation, trying number 2

[Keith Jenkins]

Although, to confuse matters, OGC Simple Feature Access seems to
specify the opposite:

> The exterior boundary LinearRing defines the “top” of the surface which
> is the side of the surface from which the exterior boundary appears to
> traverse the boundary in a counter clockwise direction. The interior
> LinearRings will have the opposite orientation, and appear as clockwise
> when viewed from the “top”,

>From section 6.1.11.1 (Polygon, Triangle description)
http://www.opengeospatial.org/standards/sfa

Keith

On Sat, Jul 16, 2016 at 2:12 PM, Keith Jenkins <kgj2 at cornell.edu> wrote:
> And exterior-ring-clockwise is how PostGIS interprets the right-hand
> rule as well:
>     http://postgis.net/docs/manual-2.2/ST_ForceRHR.html
>
> Keith
>
> On Sat, Jul 16, 2016 at 1:25 PM, Jukka Rahkonen
> <jukka.rahkonen at latuviitta.fi> wrote:
>> 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
>>>>
>>>> _______________________________________________
>>>> GeoJSON mailing list
>>>> GeoJSON at lists.geojson.org
>>>> http://lists.geojson.org/listinfo.cgi/geojson-geojson.org
>>>
>>>
>>> --
>>> Sean Gillies
>>> _______________________________________________
>>> GeoJSON mailing list
>>> GeoJSON at lists.geojson.org
>>> http://lists.geojson.org/listinfo.cgi/geojson-geojson.org
>>
>> _______________________________________________
>> GeoJSON mailing list
>> GeoJSON at lists.geojson.org
>> http://lists.geojson.org/listinfo.cgi/geojson-geojson.org