[GRASS-user] V.generalize

GRASS GRASS user
1

Dear all,

In the process of transforming a raster classification into a vector layer, I use among others the command V.GENERALIZE.

v.generalize input=v_classif_ext@PERMANENT layer=1 type=area type=boundary method=reduction threshold=4 alpha=0.5 iterations=4 output=v_classif_ext_redv.generalize input=v_classif_ext_red@PERMANENT layer=1 type=area type=boundary method=sliding_averaging threshold=100 alpha=0.5 iterations=4 output=v_smooth_1

with
grassversion=79

srid=2154

And I wonder why the smoothing result is not homogeneous, even sometimes for the same polygon.
Any idea about the reason?

Best regards

Fabrice Dazin

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2

Hi,

On Mon, Aug 24, 2020 at 4:35 PM DAZIN Fabrice (SIRS)
<Fabrice.DAZIN@sirs-fr.com> wrote:

Dear all,

In the process of transforming a raster classification into a vector layer, I use among others the command V.GENERALIZE.

v.generalize input=v_classif_ext@PERMANENT layer=1 type=area type=boundary method=reduction threshold=4 alpha=0.5 iterations=4 output=v_classif_ext_redv.generalize input=v_classif_ext_red@PERMANENT layer=1 type=area type=boundary method=sliding_averaging threshold=100 alpha=0.5 iterations=4 output=v_smooth_1

with
grassversion=79
srid=2154

And I wonder why the smoothing result is not homogeneous, even sometimes for the same polygon.
Any idea about the reason?

Not sure about this particular "reduction" method but did you already
try with a different method?

Best regards,
Markus

--
Markus Neteler, PhD
https://www.mundialis.de - free data with free software
https://grass.osgeo.org
https://courses.neteler.org/blog

3

Hi,

Yes i try with other methods of v.generalize tool with same problems.
I realize that if the generalisation tool creates a topological error on a part of the vector then the tool does not treat this part and leaves it as it is.
I didn’t succeed in forcing him to do it.

As a workaround i used https://mapshaper.org/ that implement the Visvalingam algo that work fine with a real time result!

(see https://gis.stackexchange.com/questions/87505/grass-v-generalize-method-douglas-not-working-as-expected)

My challenge is now to create a tool in grass with this Visvalingam algo.

Best regards

Fabrice Dazin

···

De : Markus Neteler neteler@osgeo.org
Envoyé : mercredi 26 août 2020 16:27
À : DAZIN Fabrice (SIRS) Fabrice.DAZIN@sirs-fr.com
Cc : GRASS user list grass-user@lists.osgeo.org
Objet : Re: [GRASS-user] V.generalize

Hi,

On Mon, Aug 24, 2020 at 4:35 PM DAZIN Fabrice (SIRS)
Fabrice.DAZIN@sirs-fr.com wrote:

Dear all,

In the process of transforming a raster classification into a vector layer, I use among others the command V.GENERALIZE.

v.generalize input=v_classif_ext@PERMANENT layer=1 type=area type=boundary method=reduction threshold=4 alpha=0.5 iterations=4 output=v_classif_ext_redv.generalize input=v_classif_ext_red@PERMANENT layer=1 type=area type=boundary method=sliding_averaging threshold=100 alpha=0.5 iterations=4 output=v_smooth_1

with
grassversion=79
srid=2154

And I wonder why the smoothing result is not homogeneous, even sometimes for the same polygon.
Any idea about the reason?

Not sure about this particular “reduction” method but did you already
try with a different method?

Best regards,
Markus

–
Markus Neteler, PhD
https://m365.eu.vadesecure.com/safeproxy/v3?f=RvgVPrGFK9ImWkc-9ucPkT9AzWPavDwg5ArpeNZO76GHNfmu9lpEhdAB9mPfAIPd&i=QLTB8Xx5rEin-ejf6tcfqHNZKtEZCTz67dt9z7tSA_LOvn7_adzZnF75USR2DAN00a-e9VK7JN6-lYZ8w2hZTQ&k=y2yg&r=HEfuyjLfWxXkfJp6GHFMLpwbQoGmoxACm10xKlkrlyQFjw7woCT5r4dUIFgcq8FO&u=https%3A%2F%2Fwww.mundialis.de - free data with free software
https://m365.eu.vadesecure.com/safeproxy/v3?f=58-viv3m7Zpv529C2dqB0DyOlDDgVIsuSqAqdZObHwfQEoUb5YWRMJyxmUPZVNpW&i=YR0xURJMeg8eaqd82mBKKAkbIOdoE-7ec56BwNI59hNv-a0JutacywyaIc3Mim0Rj_XfDpKHgjufUUek5owIQA&k=gGQO&r=QTd8p84Y91HNOcnftrP7SROBtekUcvegEtbqD7srCXHqjXgqyC_JwCaDEv-zoucw&u=https%3A%2F%2Fgrass.osgeo.org
https://m365.eu.vadesecure.com/safeproxy/v3?f=UlBtn-GsC69yUrXb8YoleYNVYehGazxYM92ZFD5292MvRueEqmNEpLZ2LIMi1nHp&i=1t2-Qxk4t1wp9x95oA6Ik3voCjEeNJDUr3iach3VFY0Exl_nYRUiOLNK4rUz8-0c19EflCq2lX_Cp8Vv7pmnsw&k=EnXZ&r=h41QrHhrJELVMIbNOiWM2PZIcDewreyiiVUdqyJ3tPeVDJ38TJH7IxO66ceBiYHz&u=https%3A%2F%2Fcourses.neteler.org%2Fblog

Ce message et toutes les pièces jointes (ci-après le “message”) sont établis à l’intention exclusive de ses destinataires et sont confidentiels. Si vous recevez ce message par erreur ou s’il ne vous est pas destiné, merci de le détruire ainsi que toute copie de votre système et d’en avertir immédiatement l’expéditeur. Toute lecture non autorisée, toute utilisation de ce message qui n’est pas conforme à sa destination, toute diffusion ou toute publication, totale ou partielle, est interdite. L’Internet ne permettant pas d’assurer l’intégrité de ce message électronique susceptible d’altération, l’expéditeur (et ses filiales) décline(nt) toute responsabilité au titre de ce message dans l’hypothèse où il aurait été modifié ou falsifié.

This message and any attachments (the “message”) is intended solely for the intended recipient(s) and is confidential. If you receive this message in error, or are not the intended recipient(s), please delete it and any copies from your systems and immediately notify the sender. Any unauthorized view, use that does not comply with its purpose, dissemination or disclosure, either whole or partial, is prohibited. Since the internet cannot guarantee the integrity of this message which may not be reliable, the sender (and its subsidiaries) shall not be liable for the message if modified or falsified.