[Qgis-user] how to simplify polygons without loosing too much information

Tue Jun 28 01:05:47 PDT 2022

Thanks very much for the responses.

One of the key features I am after is that any method does not ‘loose’ chunks of the polygon, does not matter too much if a few extra bits are included but if they are lost this is more of a problem. Looking through the 3 main methods it is not clear (to me at least) if they can be asked to avoid too much loss of this sort. In the third suggested url below the ‘best’ method does actually loose quite a chunk of a headland and in my case headlands like this are places where some of the more interesting points are likely to be.

I want to make sure dots within the polygon are included and it does not matter too much if some extra ones are included but it does matter more if some are lost.

From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Johannes Kröger (WhereGroup) via Qgis-user
Sent: Monday, June 27, 2022 3:58 PM
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] how to simplify polygons without loosing too much information

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Hey Michael,

the algorithms involved are Douglas-Peucker and Visvalingam. With those terms you might find some nice explanations and guidance online. Some quick links:

- http://bl.ocks.org/msbarry/9152218<https://eur01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fbl.ocks.org%2Fmsbarry%2F9152218&data=05%7C01%7Cmichael.dodd%40open.ac.uk%7C873e8c1316f04cd1561b08da584d8019%7C0e2ed45596af4100bed3a8e5fd981685%7C0%7C0%7C637919387128264602%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=0Go5FnUkewTywnnvjmUuJwvVtfP0mByd%2BtvkbAM3rek%3D&reserved=0>
- https://dulldrums.github.io/polygon-reduction-algorithms-demo/<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdulldrums.github.io%2Fpolygon-reduction-algorithms-demo%2F&data=05%7C01%7Cmichael.dodd%40open.ac.uk%7C873e8c1316f04cd1561b08da584d8019%7C0e2ed45596af4100bed3a8e5fd981685%7C0%7C0%7C637919387128264602%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=nTYsn%2By5oHZn3qeuZdEZwQiDL6SevMG2cXZhVsyoY%2FM%3D&reserved=0>
- https://ignf.github.io/CartAGen/docs/algorithms/line/visvalingam.html<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fignf.github.io%2FCartAGen%2Fdocs%2Falgorithms%2Fline%2Fvisvalingam.html&data=05%7C01%7Cmichael.dodd%40open.ac.uk%7C873e8c1316f04cd1561b08da584d8019%7C0e2ed45596af4100bed3a8e5fd981685%7C0%7C0%7C637919387128264602%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=PgCq8xg2ApJ2pQixEGmPMsQF%2Fi%2FVmZ5FYRGrTt599Pg%3D&reserved=0>

For your use case (keeping the overall shape) I would try Visvalingam first as that is pretty much its goal too.

Cheers, Hannes

Am 27.06.22 um 16:52 schrieb Michael.Dodd via Qgis-user:
I know there are various simplify tools in qgis but perhaps I don’t understand how to use them best. I want to simplify i.e. reduce the number of nodes in a polygon without changing the overall shape too much for country or similar outlines.  When I have tried this the simplify tends to chop off bits that a human doing a manual simplification would not, in some areas the simplification could loose loads of nodes whereas in other areas it needs to keep more of them or the overall shape will be greatly changed. Is there anyway round this or am I asking for something that is not logical?

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