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Indicate vertices order orientation in polygon part and ring geometries #9523

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Indicate vertices order orientation in polygon part and ring geometries #9523

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e0220bb
Indicate vertices order orientation
DelazJ Jan 4, 2025
86054e9
Add steps for drawing ring
DelazJ Jan 4, 2025
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37 changes: 29 additions & 8 deletions docs/user_manual/working_with_vector/editing_geometry_attributes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1177,6 +1177,14 @@ The |addPart| :sup:`Add Part` can also be used to add a geometry to a geometryle
feature. First, select the feature in the attribute table and digitize the new
geometry with the |addPart| :sup:`Add Part` tool.

.. note:: **Order of vertices in polygon parts**

Unlike the OGC standards, QGIS doesn't constrain vertices
of the exterior boundary of a polygon feature to be ordered counterclockwise.
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Exact. However, you can "fix" the case using processing

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The main point I wanted to highlight in the note is the parts being oriented the same way. I thus added the "fix" tip in another paragraph.

Thus, you can find both directions in a layer.
However, every parts of the same multipolygon feature
will have their outer vertices ordered following the same direction.

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Suggested change
You can however use the :ref:`qgisforcerhr` algorithm to constrain features of a layer
to have vertices of their outer boundaries ordered in the clockwise direction.


.. index::
single: Digitizing tools; Delete Part
Expand All @@ -1199,15 +1207,28 @@ To delete a part, simply click within the target part.
Add Ring
--------

You can create ring polygons using the |addRing|
:sup:`Add Ring` icon in the toolbar. This means that inside an existing area, it
is possible to digitize further polygons that will occur as a 'hole', so
only the area between the boundaries of the outer and inner polygons remains
as a ring polygon.
You can create ring polygons using the |addRing| :sup:`Add Ring` icon in the toolbar.
This means that inside an existing area, it is possible to digitize further polygons
that will occur as a 'hole', so only the area between the boundaries
of the outer and inner polygons remains as a ring polygon.

To add a ring:

#. Select the feature(s) to modify
#. Activate the |addRing| :sup:`Add Ring` tool
#. Draw a polygon within the selected geometries,
using the aforementioned :ref:`techniques <drawing_methods>`.
A hole appears in the selected geometries.
#. If no geometry is selected when the ring is drawn,
then a hole is added to each of the polygons the ring is drawn over.

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Here, you can add a note indicating that after AddRing, the vertices of the inner rings are oriented in the opposite direction to the exterior ring.

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Hmmm... Isn't this covered in the note just below?

.. note:: **Order of vertices in polygon rings**

.. FixMe: I think this tool should behave as below
.. Like many digitizing tools, the |addRing| :sup:`Add Ring` tool adds ring to all
.. selected features if any, otherwise all overlapping features are pierced.
Unlike the OGC standards, QGIS doesn't constrain vertices
of the exterior boundary of a polygon feature to be ordered counterclockwise.
Thus, you can find both directions in a layer.
However, every rings of the same (multi)polygon feature
will have their vertices ordered in the opposite direction to the outer boundary's.

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Suggested change
You can however use the :ref:`qgisforcerhr` algorithm to constrain features of a layer
to have vertices of their outer boundaries ordered in the clockwise direction,
and vertices of their interior rings ordered in the counter-clockwise direction.


.. index::
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