[Qgis-user] Qgis/Postgis : Multi Union (25 polygonal layers)

[celati Laurent]

 Dear all,
I'm working with QGIS and PostGIS. As input, I have 25 polygonal layers
covering a large area (multicities area). One of these data is a very large
dataset (1 million objects). The other 24 are much smaller (a maximum of a
hundred objects).
For information, I should point out that some of these polygonal datasets
are in "multi-part features" mode and others in "single-part features"
mode. I imagine this may ultimately have a slight impact on the
method/result. These 25 polygonal .shp files have highly variable,
non-homogeneous/non-harmonized data structures. Each layer has a "data_id"
field that allows  to define/link/reference, for each feature, its
membership in the layer. For example, all values in the "data_id" field for
the first layer have a value of '1'. For the second layer, the field values
are '2', etc.

My goal would be to be able to apply/adapt the existing QGIS geoprocessing
tool called "Multiple Union":
https://docs.qgis.org/3.40/en/docs/user_manual/processing_algs/qgis/vectoroverlay.html#union-multiple

Below a screenshot from the QGIS documentation :

[image: image.png]

My goal would be to have an output file:


   -  Which would be the result of the union/overlay of the 25 input data.
   To use the terms of the QGIS documentation, the processing should check for
   overlaps between features within the 25 layers and create separate features
   for the overlapping and non-overlapping parts. This "multiple union"
   geoprocessing seems interesting for my goal where there is no overlap (a,
   NULL; b, NULL; c, NULL).


   - For areas where there is an overlap, the QGIS union geoprocessing
   creates as many identical overlapping features as there are features
   participating in this overlap. This doesn't bother me. But since,
   ultimately, I'd like a field in the result/output file to allow, for each
   feature, to retrieve the list of input layers that participate/contribute
   to this result feature (in order to retrieve the origin/source of the
   data). I was wondering/thinking it might be better if only one feature was
   created per overlapping area?


   -  I'd like a field in the result file to allow, for each feature, to
   retrieve the list of input layers that participate/contribute to this
   result feature. In order to retrieve the origin/source of the data.


   - Ideally, a field that allows you to retrieve the number (COUNT) of
   layers that contribute to this feature (at least 1 layer, at most 25
   layers).


   - Regarding the non-geometric attributes/fields, I would like to be able
   to specify the selection/define the list of fields I ultimately want to
   keep. I don't want to keep all of the fields, but rather just some of the
   fields for each of the 25 input layers.


I imagine it's recommended to do this processing in PostGIS rather than
QGIS? I can, if necessary, import my 25 SHP files into a PostGIS database.
I also imagine it's important to keep in mind that the "multi-part
features" / "single-part pieces/features" mode of the input layers can
affect the result. If I'm using a PostGIS query, I was thinking it might be
helpful to force all features to be in single-part mode (using the PostGIS
'st_dump' function?).

In advance, Thanks so much for your help, guidance.
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