[Qgis-user] Simplify?

Tue Nov 15 04:59:34 PST 2011

Hi Roger

Thanks for your comments. Much appreciated!

Chris

From: Roger André [mailto:randre at gmail.com] 
Sent: 14 November 2011 21:50
To: Chris Green
Cc: tech at wildintellect.com; qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Simplify?

GRASS has some very good line simplification options which I believe can be accessed from within QGis.  I like to run a light Douglas Peucker simplification to cut the vertice count down without dramatically altering the appearance of the data, followed by Lang simplification to generalize the appearance of things.  A good description of the difference between these algorithms can be found at http://psimpl.sourceforge.net/douglas-peucker.html and http://psimpl.sourceforge.net/lang.html.

BTW, using a vary small threshold tolerance with Douglas Peucker is a great way to remove vertices in a straight-line segments.  For example, running a tolerance of .0015 on a set of fairly complex world country borders in GRASS yields the following for me:

 Number of vertices was reduced from 606998 to 230824

This pruning is not discernible against the original data set until about 1.250000 map scale.  At 1:50000 it starts to look a little blocky.

MapShaper is a great tool, and by doing a pruning of your excess vertices as metioned above, it might put the data under the 80 MB limit and allow you to use it.

Roger

--

On Mon, Nov 14, 2011 at 1:40 AM, Chris Green <chris.green at ibstv.co.uk> wrote:

Hi Alex

Thanks for your response. I have tried Mapshaper on smaller shp files, it is pretty good but it has a limit of 80M and some of my files are too large for this.

I would still very much like to get the Simplify function working. I assume from your comments that in general the QGIS Simplify function works fine so either I am doing something wrong, or there is a problem with my local copy of QGIS which I could resolve be re-installing?

By the way I can't find the DPSimplify plugin, any idea where I might find it?

Chris

-----Original Message-----
From: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Alex Mandel
Sent: 11 November 2011 20:27
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Simplify?

I see at least 3 tools in QGIS to do simply.
The one you mentioned in the vector menu (part of ftools)
The DPSimplify plugin (appears to be the precursor to the above tool)
and the Simply function in the vector editor

Of those the last one is the one I've used the most but operates on
individual features at a time. The key question I can see for all of
them though is what tolerance are you setting and what projection is
your data in since this determines the units for tolerance.

I just tested with a tolerance of 1 on a GCS Lat/Lon WGS84 dataset of
lines and it seemed to work fine, I imagine if I had left the tolerance
at 0.0001 it would have done nothing.

Worst case scenario mapshaper is an online tool just for this.

Enjoy,
Alex

On 11/11/2011 05:39 AM, Chris Green wrote:
> Hi Ramon
>
>
>
> I have been trying to modify the original shape file rather than save to a new file. When I tried selecting the ‘save to a new file’ option , the message after simplification said
>
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> ‘There were 3638321 vertices in original dataset which were reduced to 3638321 vertices after simplification’
>
>
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> In other words, no simplification at all. I suspect that this is actually what is happening when I try to over-write the existing file – it indicates that simplification took place but actually what is saved is identical to the original.
>
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> Thanks anyway…..
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> Chris
>
>
>
> From: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Ramon Andiñach
> Sent: 11 November 2011 13:19
> To: <qgis-user at lists.osgeo.org>
> Subject: Re: [Qgis-user] Simplify?
>
>
>
>
>
> On 11/11/2011, at 20:40, "Chris Green" <chris.green at ibstv.co.uk> wrote:
>
> Hello
>
>
>
> I am using QGIS 1.7 in a Windows 7 environment. Everything seems great except for the simplify function (Vector->Geometry Tools->Simplify Geometries)
>
>
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> I am trying to simplify an ESRI shape file which is around 95M. I have tried various tolerance settings, but although it looks like some simplification is taking place, actually the file is unchanged. So for example with tolerance setting of 1.0 I get an encouraging message that says:
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>
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> ‘There were 3638321 vertices in original dataset which were reduced to 559351 vertices after simplification’
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> However after saving the changes to the layer, the dataset still looks identical to the un-simplified version and the file size remains unchanged at 95M. I have tried to simplify smaller shape files also, but with the same result.
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> Can someone tell me what I am I doing wrong?
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> Hi Chris,
>
> This is a guess...
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> I suppose your output file is different to your input file?
>
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> (otherwise windows will probably get confused)
>
>
>
> -ramon.
>

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