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Is it possible to smooth or generalize a polygon in qgis? Ìs there a plugin available like the generalizer-plugin for lines? I could convert my polygons to lines, smooth them and convert them back to polygones. But this is not working for my polygons because of sveral reasons.

Any idea? Thanks!

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2 Answers 2

up vote 6 down vote accepted

you can use:

Vector menu -> Geometry tools -> Simplify Geometries



Beside this you can use Douglas-Peucker algorithm in postgis too, so you can use in postgis then adding postgis layer to qgis. you can find some info here about Simplify: Reduce the weight of a geometry.

SELECT simplify(the_geom,500) as simpgeom 
    FROM neighborhoods


i hope it helps you...

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That simplifies the lines, making them more jagged. Is there a way to smooth them(which I guess would have to interpolate some points)? –  Nate Wessel Dec 23 '12 at 17:13
Simplify just removes vertices from the line or polygon, which can still leave jagged lines. You want the Generalizer plug in if you want smoothed lines. –  MicheleTobias yesterday

I've also had luck using the QGIS Generalizer plugin (enable experimental plugins), and then:

Vector > Geometry Tools > Polygons to lines

Then use the Generalizer plugin to smooth the lines and vertices

Plugins > Generalizer > Generalizer
Algorithm: "Chaiken's Algorithm"

Then turn the lines back into a polygons

Vector > Geometry Tools > Polygons to lines

This does a better job actually smoothing the lines and rounding the corners. It's a bit clunky to have to have to switch back and forth between lines and polys, but it works. I know that there's ways to do this via Grass as well, but I've found them difficult to setup.

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