[Qgis-user] QGIS - Vector layer with multiple entities

[Edouard Bourgine]

Hi,

 

I’m running qgis 3.16 on w10.

 

I have two vector layers with multiple entities :

·         One with each town borders (? I’m note sure if this is the right
word) I mean the area for each zip code. (TOWN_LAYER) (polygon vector)

·         The second with cropfields and meadows (polygon vector)
(FIELD_LAYER)

 

I want the total area of cropfield and Meadows in each Town from the first
layer. (Exemple : one town have 31 fields/meadows -> total town area is
55km² (i have that info) and cropfield/meadows area is 26km² (i need that
one for each town)

 

First i tried something like :

·
Sum(area(intersection($geometry,geometry(‘FIELD_LAYER_ID’))),group_by
:=FIELD WITH SAME VALUE EVERYWHERE)

 

But it is not a valid expression (geometry(‘FIELD_LAYER_ID’) don’t work)

 

Some meadows span over several town. Some juste inside one. 

I’ve « cut » the FIELD_LAYER with the TOWN_LAYER

 

My first vector layer as a field specific and i’m looking for the formula
that would give me the right answer.

 

I’ve tried :

 

·         aggregate (layer :=’FIELD_LAYER_ID’, aggregate :=’sum’,expression
:=$area, filter := intersects($geometry, geometry(@parent)))

 

It doesn’t work because the borders are overlapping : each fields just
touching the borders even if they belong to another town are included.

 

« Contains » instead of « intersects » doesn’t work either because some
borders of the field are overlapping and those are not included.

 

Exemple with « count » instead of « sum »

 

A town have 31 fields spanning over 26km² : Intersects give me a total count
of 38 and 48km² and contains give me 12 and 4,7 km² because of borders
overlap.

 

I think the solution is with intersection, but i can’t reach a valid
expression.

 

Another solution may be splitting the FIELD_LAYER in as much layer as town
(I did that) and batch process for each layer :

·         sum($area,group_by := FIELD WITH SAME VALUE EVERYWHERE)

 

then get the result in the TOWN_LAYER. 

 

But i don’t know how to batch process it and get the result in the first
layer.

 

And for informations I’m talking hundreds of towns and thousands of
fields/meadows entities so one by one calculation is not possible.

 

I hope I make sense and that someone know how to deal with that.

 

Thanks,

 

Edouard BOURGINE

Ingénieur Environnement

 

Tél/Fax : 03.83.22.69.09

Port : 06.63.12.99.63

 

ACOSOL

26 rue de l’Ornain

54520 LAXOU

 



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