<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Good description of a reasonable approach Brent! At one point, for a certain dataset, I was able to calculate a pretty reliable (e.g. still made valid features) ratio of simplification threshold based on map scale. I don't have the data anymore, but it was almost like an LOD level of detail approach. <div><br></div><div>With that in mind, I'd like to see what one could do with LOD approach in OpenGL with map data - probably blows away our best little hacks ;-) Never got to that yet :)<div><br><div><div>On 2012-05-25, at 4:26 PM, <a href="mailto:pcreso@pcreso.com">pcreso@pcreso.com</a> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><table cellspacing="0" cellpadding="0" border="0"><tbody><tr><td valign="top" style="font: inherit;">My 02c...<br><br>If you have a table with a geometry column, you can add a second geometry column for the simplified ones. As in the script below, for a national coastline with about 10,000 polygons<br><br>You can generate the simplified ones using a series of update SQL statements, based on a variety of criteria, including neighbouring (or not) via a ST_Distance() in the where clause.<br><br>If you hand craft the update SQL's, you can just copy rather than simplify smaller polygons (ST_Area in the where clause), etc.<br><br>A slowish way, but viable for a 1-off is to have a where clause with an ST_IsValid in the where clause testing the result of the simplification. If the where clause includes a test for the target geometry being not-null, the series can apply a large simplification initially, then progressively apply a decreasing simplifcation buffer to
features that failed previous iterations, until the last statement copies any remaining features with zero simplification.<br><br>I prefer a script with a set of SQL's rather than a function, as it is easier to tweak case requirements. <br><br>The advantage of this approach over the preserving topology simplify function is the degree of control the hand crafted SQL's provide over exactly how to simplify the dataset.<br><br>But, as always, your mileage may vary :-)<br><br> Cheers,<br><br> Brent Wood<br><br><br>#! /bin/bash<br>#<br># script to simplify NZ coastline for zoom layer<br><br>DB=nzcoast<br>TAB=nz_all<br><br><br># add new geometry column<br>psql -d $DB -c "alter table nz_all drop column simple_geom;"<br><br>psql -d $DB -c "alter table nz_all add column simple_geom geometry(multipolygon,4326);"<br><br><br># populate<br># do small islands with no simplification<br>psql -d $DB -qc "update
nz_all<br> set simple_geom=geom <br> where ST_Area(geom) < 0.0000000001;"<br>psql -d $DB -qc "update nz_all<br> set simple_geom=ST_Simplify(geom, 0.001) <br> where simple_geom isnull<br> and ST_IsValid(ST_Simplify(geom, 0.001));"<br>psql -d $DB -qc "update nz_all<br> set simple_geom=ST_Simplify(geom, 0.0001) <br> where
simple_geom isnull<br> and ST_IsValid(ST_Simplify(geom, 0.0001));"<br>psql -d $DB -qc "update nz_all<br> set simple_geom=ST_Simplify(geom, 0.00001) <br> where simple_geom isnull<br> and ST_IsValid(ST_Simplify(geom, 0.00001));"<br>psql -d $DB -qc "update nz_all<br> set simple_geom=geom <br> where simple_geom isnull;"<br><br><br><br><br>--- On <b>Sat, 5/26/12, Carson Farmer
<i><<a href="mailto:carson.farmer@gmail.com">carson.farmer@gmail.com</a>></i></b> wrote:<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px;"><br>From: Carson Farmer <<a href="mailto:carson.farmer@gmail.com">carson.farmer@gmail.com</a>><br>Subject: Re: [Qgis-developer] Vector Layer Generalization<br>To: "Evgeniy Pashentsev" <<a href="mailto:ugnpaul@gmail.com">ugnpaul@gmail.com</a>><br>Cc: <a href="mailto:qgis-developer@lists.osgeo.org">qgis-developer@lists.osgeo.org</a><br>Date: Saturday, May 26, 2012, 10:11 AM<br><br><div class="plainMail">Excellent! Topology preserving simplification is something I've been<br>looking for!<br>> But I want to ask your advice on one thing. Is it normal for user<br>> to simplify all the features on a layer at once? I'm just not sure<br>> which way is more suitable for a regular user:<br>> - simplify all the features at once as it is now in my current prototype;<br>> - simplify selected feature and all the corresponding vertexes of<br>> the neighboring features;<br>> - or simplify only selected
features;<br>> - or even something else.<br>> Although it is not very important for current work, I want to make sure<br>> I don't implement useless functionality.<br>Personally, I normally simplify an entire layer at once... for mapping<br>purposes, or simply to reduce drawing time or for computational/space<br>efficiency. It might be nice to also have the option to simplify<br>selected features only, but if this was an option, then I would expect<br>*only* those selected features to be simplified... and *not* their<br>neighbouring features...<br><br>That's my two cents anyway, looking forward to trying it out!<br><br>Carson<br><br><br>-- <br>Dr. Carson J. Q. Farmer<br>Centre for GeoInformatics (CGI)<br>School of Geography and Geosciences<br>Irvine Building, University of St Andrews<br>St Andrews, Fife, KY16 9AL<br>Scotland, UK<br>_______________________________________________<br>Qgis-developer mailing list<br><a ymailto="mailto:Qgis-developer@lists.osgeo.org" href="x-msg://1873/mc/compose?to=Qgis-developer@lists.osgeo.org">Qgis-developer@lists.osgeo.org</a><br><a href="http://lists.osgeo.org/mailman/listinfo/qgis-developer" target="_blank">http://lists.osgeo.org/mailman/listinfo/qgis-developer</a><br></div></blockquote></td></tr></tbody></table>_______________________________________________<br>Qgis-developer mailing list<br><a href="mailto:Qgis-developer@lists.osgeo.org">Qgis-developer@lists.osgeo.org</a><br><a href="http://lists.osgeo.org/mailman/listinfo/qgis-developer">http://lists.osgeo.org/mailman/listinfo/qgis-developer</a><br></blockquote></div><br><div>
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