[Qgis-user] (re)colloring overlapping parts of multipolygons

[chris hermansen]

I second Nicolas' IDW suggestion though since a point source signal
strength is proportional to the inverse square distance you might want to
take that into account

On Sat, Jun 19, 2021, 10:57 Nicolas Cadieux <njacadieux.gitlab at gmail.com>
wrote:

> Hi,
> There are multiple ways of doing this.  The Join attributes by location
> (Summary) could help you could the number of polygons overlapping the
> point.  You could then split the overlapping  circles into smaller parts
> and then use the join by location again to transfer the sums polygon of the
> points back to the various circle parts.  Then use the symbology by
> classification using this field.
>
> A much better way would be to make an interpolation of your point data
> instead. The easy way would be just to make a heat map.  The best way would
> probably be to use IDW with a search radius.  The gdal version of idw found
> in processing will permit you to do this.  Find an interpolation method
> that is compatible with the phenomenon you are studying. For example  IDW
> is would be better that a tin or a voronoï polygon.
>
> Nicolas Cadieux
> https://gitlab.com/njacadieux
>
> Le 19 juin 2021 à 10:54, Richard Duivenvoorde <rdmailings at duif.net> a
> écrit :
>
> 
>
> Hi List,
>
> I'm experimenting to create 'flower'-shaped polygons for a point layer,
> denoting the signal coverage of several directional radio transmitters on a
> pole (fyi: to find/locate bats with small transmitters).
>
> After some fiddling I came to the attached result for a point with 3 (but
> dynamic from attribute) transmitters using the following expression:
>
> collect_geometries(
> array_foreach( string_to_array("azimuths"),
> make_circle( make_point(
>        $x + "bereik_m"/2 * cos(radians(@element)),
>        $y + "bereik_m"/2* sin(radians(@element))), "bereik_m" )
>        ))
>
> The result being a multipolygon with overlapping parts.
>
> In this work the overlapping area'a are (apparently) the most interesting,
> as there the 'location' accuracy is highest.
>
> So my question: is there a way to give the overlapping area's really
> different colors.
> I've been playing with the layer rendering options, but ideally the area's
> in which there is no, 2 parts, 3 parts, and 4 parts overlapping should be
> differently colored..
>
> I could try to create more advanced expressions (or probably I would go
> for some python) for that, but I was hoping maybe there is another
> trick/idea to use current multi-polygons, without adding 'parts'
>
> Any help/tip is appreciated.
>
> Regards & TIA,
>
> Richard Duivenvoorde
> <multipolygon.jpeg>
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