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.gr...@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-boun...@lists.osgeo.org [mailto: > qgis-user-boun...@lists.osgeo.org] On Behalf Of Alex Mandel > Sent: 11 November 2011 20:27 > To: qgis-user@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-boun...@lists.osgeo.org [mailto: > qgis-user-boun...@lists.osgeo.org] On Behalf Of Ramon Andiñach > > Sent: 11 November 2011 13:19 > > To: <qgis-user@lists.osgeo.org> > > Subject: Re: [Qgis-user] Simplify? > > > > > > > > > > > > On 11/11/2011, at 20:40, "Chris Green" <chris.gr...@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@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/qgis-user > > > _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/qgis-user >
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