[GeoJSON] polygon orientation, trying number 2

[Keith Jenkins]

Okay, my last post for today...

The answers here provide a good overview of the messy situation:
http://gis.stackexchange.com/questions/119150/order-of-polygon-vertices-in-general-gis-clockwise-or-counterclockwise

Keith

On Sat, Jul 16, 2016 at 2:19 PM, Keith Jenkins <kgj2 at cornell.edu> wrote:
> 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
>>>> _______________________________________________
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>>>
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