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Status: Reported (1)

RFC 7946, "The GeoJSON Format", August 2016

Source of RFC: geojson (art)

Errata ID: 5069

Status: Reported
Type: Technical

Reported By: Clark Archer
Date Reported: 2017-07-14

Section 3.1.6 says:

3.1.6.  Polygon

   To specify a constraint specific to Polygons, it is useful to
   introduce the concept of a linear ring:

   o  A linear ring is a closed LineString with four or more positions.

   o  The first and last positions are equivalent, and they MUST contain
      identical values; their representation SHOULD also be identical.

   o  A linear ring is the boundary of a surface or the boundary of a
      hole in a surface.

   o  A linear ring MUST follow the right-hand rule with respect to the
      area it bounds, i.e., exterior rings are counterclockwise, and
      holes are clockwise.

   Note: the [GJ2008] specification did not discuss linear ring winding
   order.  For backwards compatibility, parsers SHOULD NOT reject
   Polygons that do not follow the right-hand rule.

   Though a linear ring is not explicitly represented as a GeoJSON
   geometry type, it leads to a canonical formulation of the Polygon
   geometry type definition as follows:

   o  For type "Polygon", the "coordinates" member MUST be an array of
      linear ring coordinate arrays.

   o  For Polygons with more than one of these rings, the first MUST be
      the exterior ring, and any others MUST be interior rings.  The
      exterior ring bounds the surface, and the interior rings (if
      present) bound holes within the surface.

It should say:

3.1.6.  Polygon

   To specify a constraint specific to Polygons, it is useful to
   introduce the concept of a linear ring:

   o  A linear ring is a closed LineString with four or more positions.

   o  The first and last positions are equivalent, and they MUST contain
      identical values; their representation SHOULD also be identical.

   o  A linear ring is the boundary of a surface or the boundary of a
      hole in a surface.

   o  A linear ring MUST follow the right-hand rule with respect to the
      area it bounds, i.e., exterior rings are clockwise, and holes are
      counterclockwise.

   Note: the [GJ2008] specification did not discuss linear ring winding
   order.  For backwards compatibility, parsers SHOULD NOT reject
   Polygons that do not follow the right-hand rule.

   Though a linear ring is not explicitly represented as a GeoJSON
   geometry type, it leads to a canonical formulation of the Polygon
   geometry type definition as follows:

   o  For type "Polygon", the "coordinates" member MUST be an array of
      linear ring coordinate arrays.

   o  For Polygons with more than one of these rings, the first MUST be
      the exterior ring, and any others MUST be interior rings.  The
      exterior ring bounds the surface, and the interior rings (if
      present) bound holes within the surface.

Notes:

This is only for the bullet point describing the right-hand rule for linear rings. It seems like the clockwise/counterclockwise descriptions are the opposite of the right-hand rule. Walking an exterior ring in a counterclockwise direction would have the exterior of the ring to the right of the observer.

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