[Qgis-user] How to create polygons from areas of overlap?

[Nikos Alexandris]

* Martino <martin.pescador at inventati.org> [2017-09-28 13:52:25 +0100]:

Ciao Martino,

>I have a vector layer containing polygons that represent vegetation
>patches. There are 15 vegetation types (and most have more than one
>polygon; i.e. the polygons are multipart). Some of the polygons overlap
>as the areas have more than one classification (e.g. a polygon
>representing one plant species and a polygon representing another plant
>species).

Can you derive as many independent layers as the number of vegetation
types?  Manually or better with a small script.


>I would like to produce a new layer in which all of the overlapping
>areas become new polygons. For example, an area of overlap of a patch
>of Species A and a patch of Species B would become a new polygon
>representing areas containing both species (and the non-overlapping
>areas would become separate polygons).

I think having independent layers would be the basis to work on and
produce the kind of combinatory polygons you are aiming at.


>So I start with 
>
>Polygon with Species A
>Polygon with Species B
>
>and the result would be
>
>Polygon with only Species A
>Polygon with only Species B
>Polygon with Species A & B
>
>I am unable to figure out how to do this!
>
>If anybody has any suggestions, I would welcome them.
>
>With thanks.


If you have Polygon "A" and Polygon "B", then look after geoprocessing
operations. For example, 'Intersection', Union', and the likes should
give what you are looking for.

Alternatively, if you could work with raster maps, you can think of
having raster maps for each individual vegetation type, then combine the
maps based on some classification scheme that does include combinations
of vegetation types. Say vegetation type A is going to be assigned a
pixel value of 1, vegetation type B is going to get a pixel value
of 2 and then a pixel occupied by A and B is goint to get a pixel value
of 3.

This is an idea to approach the question.

Regards, Nikos
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-- 
Nikos Alexandris | Remote Sensing & Geomatics
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