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Is it possible to simplify just the selected parts of a polygon? I am trying to make a simplified version of the Australian Local Government Areas (LGAs) shapefile.

I am using the v.generalise function from GRASS in QGIS, which works really well for internal boundaries. However, it often fails to simplify complex coastlines, and I'm left with hundreds of LGAs that area nicely simplified down to a few tens of points, and a handful of polygons with 10s of thousands of points, nearly all of which area along complex coastlines.

A quick solution to this problem would be if I could simply select the non-shared points along each of the complex coastlines, and use QGIS's less careful Simplify function, but it doesn't seem like it is possible to use that on only part of a polygon. I guess I could also break some features apart, simplify the relevant parts, and re-join them, but I'm not sure what would happen to my metadata then.

Anyone have any ideas?

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  • Degenerate the polygons to lines, simplify the lines that are still too busy then recombine into new polygons, attributed from the centroids of the original polygons. Another method would be to grab some simplified geometries and snap the polygons to the simple geometries gis.stackexchange.com/questions/141720/…. finally you could convert your polygons to raster with a moderate cell size, convert raster to polygon, values from centroids.
    – Michael Stimson
    Commented Sep 24, 2020 at 7:11
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    Oh wow... I just realised that I can use v.generalise to v.generalise to say, 0.001°, and then use edit -> simplify feature to simplify the individually still-complex features using a finer scale (e.g. 0.0001°), and it will pretty much only simplify the complex coastlines, because the other lines are already simplified..
    – naught101
    Commented Sep 24, 2020 at 11:57

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