[GRASS-user] Number of centroids outside area: difficulty with cleaning

[Markus Metz]

On Sun, Apr 14, 2013 at 11:02 AM, RichardC <richtcooper at hotmail.com> wrote:
> Hi,
>
> I'm trying to remove a 'Number of centroids outside area' after
> re-projecting a file.
>
> I've tried v.clean with various tools. including breaklines/boundaries,
> break polygons, snap lines/boundaries.
>
> The intial imported shapefile did not display any errors, but the following
> errors appeared on re-projecting the latter file:
>
> Building topology for vector map <as_bas_shp>...
> Registering primitives...
> 44546 primitives registered
> 1680359 vertices registered
> Building areas...
> 13602 areas built
> 2828 isles built
> Attaching islands...
> Attaching centroids...
> Number of nodes: 33748
> Number of primitives: 44546
> Number of points: 0
> Number of lines: 0
> Number of boundaries: 30931
> Number of centroids: 13615
> Number of areas: 13602
> Number of isles: 2828
> *Number of centroids outside area: 13
> Number of duplicate centroids: 108
> Number of areas without centroid: 108*
>
> I can remove the duplicate centroids with v.clean (using bpol) but not the
> centroids outside area.
>
> What would be the easiest way to remove, or at least to identify (filter
> for) and manually assess?

The reason for these problems is one step earlier. Try to densify the
vector boundaries with v.split, optionally followed by
v.build.polylines before reprojecting it. See [0] for an example of
the effect on reprojection.

Markus M

[0] http://trac.osgeo.org/grass/attachment/ticket/1918/dense_points_effect.png

>
> I've also tried outputting the errors using the error= command, but no
> geometries are visible in the latter.
>
> Cheers,
> Richard
>
>
>
> --
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