[GRASS-user] v.patch eats my large polygons

[Markus Metz]

On Tue, Dec 18, 2018 at 2:14 PM Robert Nuske <rsn.mailinglists at gmail.com>
wrote:
>
> Dear GRASS-Users,
>
> after an abstinence period I started again to use GRASS 7.4 on Ubuntu
> 18.04 and am now facing v.patch eating my large polygons.
>
>
> 38 vector maps (polygon) cover separate parts of my study area. A few
> share a boundary with their neighbor, most do not. To further work with
> all polygons from all sub plots, I want to merge all sub plots into one
> vector map. I do NOT need the polygons to get dissolved (disected &
> rebuild) at the boundary of two sub plots.
>
> Large (area, number of vertices and number of islands) polygons, which
> touch another large polygon at the common border of two sub plots are
> not in the patched vector map.
>
>
> What I did:
> v.patch in=gaps_1,gaps_2 out=gaps_merged
>
> What did not help was the -b flag. Lot's of boundaries but no areas
anymore.

>From the manual:
Boundaries may need to be cleaned with *v.clean tool=break,rmdupl,rmsa*
repeatedly until the *rmsa* tool (Remove small angles at nodes) no longer
modifies any boundaries. If vector topology is still not clean, boundaries
may also need to be snapped with *v.clean tool=snap,break,rmdupl*.

v.clean on the patched vector should restore your polygons.

We could add the cleaning step to v.patch (a new enhancement ticket could
be created).

HTH,

Markus M
>
>
> To illustrate my problem, an image (https://imgur.com/4n3le65) with two
> sub plots (left an right) with a common border at the smallest part. All
> colored area are polygons. The islands are white. The orange polygons
> are the results of v.patch. The light gray and dark gray polygon are
> missing in the resulting v.patch-map.
>
>
> Did anyone experience similar behavior before and know of a cure?
>
>
> Thanks
>    Robert
>
>
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