v.generalize

[Paolo Cavallini]

Hi all.
I'm studying v.generalize; in the past I obtained good results, but now I
keep on getting either 100% or 0% of the original vertices, no matter of
which algorhitm and parameters I select: could this be a bug, or am I doing
something wrong?
Testing on debian testing, grass64-5 from deb unstable, through qgis
interface.
Thanks.
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[Paolo Cavallini]

On Tue, 03 Nov 2009 11:59:34 +0100, Paolo Cavallini <[hidden email]>
wrote:
> Hi all.
> I'm studying v.generalize; in the past I obtained good results, but now I
> keep on getting either 100% or 0% of the original vertices, no matter of
> which algorhitm and parameters I select: could this be a bug, or am I
doing
> something wrong?
> Testing on debian testing, grass64-5 from deb unstable, through qgis
> interface.
> Thanks.

E.g.:
v.generalize input=south1@butta type=area layer=1 method=douglas_reduction
threshold=1000.0 look_ahead=7 reduction=50 slide=0.5 angle_thresh=3
degree_thresh=0 closeness_thresh=0 betweeness_thresh=0 alpha=5 beta=5
iterations=1 layer=1 output=snake_simpl
Generalization (douglas_reduction)...
Building topology for vector map <snake_simpl>...
Registering primitives...
1000
1488 primitives registered
31776 vertices registered
Building areas...
713 areas built
618 isles built
Attaching islands...
Attaching centroids...
Number of nodes: 1393
Number of primitives: 1488
Number of points: 0
Number of lines: 0
Number of boundaries: 775
Number of centroids: 713
Number of areas: 713
Number of isles: 618
Number of vertices was reduced from 31776 to 31776 [100%]

Finalizado com sucesso

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[Daniel Bundala]

Hello Paolo,

Try v.build.polylines. This was discussed couple of times before. So
you might want to have a look at some older discussions.

Daniel

On Tue, Nov 3, 2009 at 11:07 AM, Paolo Cavallini <[hidden email]> wrote:

>
> On Tue, 03 Nov 2009 11:59:34 +0100, Paolo Cavallini <[hidden email]>
> wrote:
>> Hi all.
>> I'm studying v.generalize; in the past I obtained good results, but now I
>> keep on getting either 100% or 0% of the original vertices, no matter of
>> which algorhitm and parameters I select: could this be a bug, or am I
> doing
>> something wrong?
>> Testing on debian testing, grass64-5 from deb unstable, through qgis
>> interface.
>> Thanks.
>
> E.g.:
> v.generalize input=south1@butta type=area layer=1 method=douglas_reduction
> threshold=1000.0 look_ahead=7 reduction=50 slide=0.5 angle_thresh=3
> degree_thresh=0 closeness_thresh=0 betweeness_thresh=0 alpha=5 beta=5
> iterations=1 layer=1 output=snake_simpl
> Generalization (douglas_reduction)...
> Building topology for vector map <snake_simpl>...
> Registering primitives...
> 1000
> 1488 primitives registered
> 31776 vertices registered
> Building areas...
> 713 areas built
> 618 isles built
> Attaching islands...
> Attaching centroids...
> Number of nodes: 1393
> Number of primitives: 1488
> Number of points: 0
> Number of lines: 0
> Number of boundaries: 775
> Number of centroids: 713
> Number of areas: 713
> Number of isles: 618
> Number of vertices was reduced from 31776 to 31776 [100%]
>
> Finalizado com sucesso
>
> --
> http://faunalia.it/pc
> _______________________________________________
> grass-dev mailing list
> [hidden email]
> http://lists.osgeo.org/mailman/listinfo/grass-dev
>

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[Martin Landa]

Hi,

2009/11/3 Daniel Bundala <[hidden email]>:
> Try v.build.polylines. This was discussed couple of times before. So
> you might want to have a look at some older discussions.

probably it would be good to document it or at least to mention it in
the manual/tutorial of v.generalize (if it's not).

Martin

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[Paolo Cavallini]

Hi Daniel.
Thanks for your reply. I'm trying to generalize (and smooth) polygons, not
lines, so your suggestion do not apply.
Any other hint?
All the best.

On Tue, 3 Nov 2009 18:20:39 +0000, Daniel Bundala <[hidden email]>
wrote:

> Try v.build.polylines. This was discussed couple of times before. So
> you might want to have a look at some older discussions.
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[Markus Metz-2]

Paolo Cavallini wrote:
> Hi Daniel.
> Thanks for your reply. I'm trying to generalize (and smooth) polygons, not
> lines, so your suggestion do not apply.
>  
No, Daniel's suggestions do apply because you want to generalize
boundaries. If an area is defined by many short boundaries, there is
nothing to smooth because each boundary is processed separately and you
need to build polylines first in order to get nice results. See previous
posts on this topic.

Also type=boundary instead of type=area would help.

Markus M

> Any other hint?
> All the best.
>
> On Tue, 3 Nov 2009 18:20:39 +0000, Daniel Bundala <[hidden email]>
> wrote:
>
>  
>> Try v.build.polylines. This was discussed couple of times before. So
>> you might want to have a look at some older discussions.
>>    

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