[GeoJSON] polygon orientation, trying number 2

[Sean Gillies]

Dear all,

I am so grateful that Anna caught the problem with the examples. But the
rest of this discussion is moot. The GeoJSON WG isn't going to revisit any
of the specification before it is published unless the RFC Editor finds a
technical defect.

Yours,

On Sat, Jul 16, 2016 at 11:25 AM, Keith Jenkins <kgj2 at cornell.edu> wrote:

> 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
> >>>> _______________________________________________
> >>>> GeoJSON mailing list
> >>>> GeoJSON at lists.geojson.org
> >>>> http://lists.geojson.org/listinfo.cgi/geojson-geojson.org
> >>>
> >>> _______________________________________________
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>



-- 
Sean Gillies



-- 
Sean Gillies
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