[Qgis-user] Simplify?

Mon Nov 14 13:50:23 PST 2011

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
> >
> >
> >
> > ‘There were 3638321 vertices in original dataset which were reduced to
> 3638321 vertices after simplification’
> >
> >
> >
> > 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.
> >
> >
> >
> > Thanks anyway…..
> >
> >
> >
> >
> >
> > 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)
> >
> >
> >
> > 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:
> >
> >
> >
> > ‘There were 3638321 vertices in original dataset which were reduced to
> 559351 vertices after simplification’
> >
> >
> >
> > 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.
> >
> >
> >
> > Can someone tell me what I am I doing wrong?
> >
> >
> >
> >
> >
> > Hi Chris,
> >
> > This is a guess...
> >
> >
> >
> > I suppose your output file is different to your input file?
> >
> >
> >
> > (otherwise windows will probably get confused)
> >
> >
> >
> > -ramon.
> >
>
>
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-user
>
>
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-user
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20111114/197f0e9f/attachment.html>