[Qgis-user] Duplicate Points from Extract Nodes

[Goyo]

2013/6/11 magerlin <MAP at ramboll.dk>:
> Is it intentional that when I use "Vector", "Geometry Tools", "Extract Nodes"
> on a rectangle shaped polygon I get 5 points?
>
> It seems like Qgis counts both a starting point and an ending point even
> though they are identical?
> <http://osgeo-org.1560.x6.nabble.com/file/n5059332/5Points.png>
>
> I have checked that my polygon really only has 4 points (no "double points"
> which you can sometimes make unintentionally when digitising by the mouse).

I think it's just trying to follow OGC Simple Features Specification.
According to it the first and last vertices of a closed ring must be
"doubled" and polygons are defined by closed rings. Sort of, I am no
expert on that matter.

Select a rectangle, copy and paste into a text editor and you will see
the WKT version of the geometry with five points, the first and last
ones identical.

Not sure, maybe it would make sense for the "extract nodes" tool to
discard the last vertex in every ring of a polygon?

Goyo