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Can I subtract two attributes belonging  to two different layers? In this case these would be two polygons holding DEPTH attribute (Depth1 - Depth2).   Is this possible and if so how?

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  • 2
    You could join attributes by location and then perform the substraction on the joined layer. Or you could create a virtual layer.
    – Erik
    Commented Jan 3, 2021 at 21:40
  • How are the polygons related, do they share some common attribute, do they overlap?
    – Bera
    Commented Jan 4, 2021 at 8:57

2 Answers 2

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Another possibility is to use a virtual layer. Let's suppose you have two layers polygon_1 and polygon_2 with fields and values as follows:

enter image description here

Go to Menu Layer / Create layer / New Virtual Layer… and paste this query:

SELECT p1.fid, depth_1-depth_2 as difference, p1.geometry
FROM polygon_1 as p1, polygon_2 as p2
where p1.fid = p2.fid

The features are connected with their common fid. You not not state how the features should be connected, so I guess you have a common identifier in your data. However, you can adapt this to your needs. The attributes table of the new virtual layer looks like this, containing a newly calculated column for the difference depth_1 - depth_2:

enter image description here

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One possibility is to use QGIS expressions. If you are in the attribute table of a layer, open the field calculator and paste the following expression (adapting it to the names of your layers/fields) to get a value from an attribute field of another layer:

attribute (
    get_feature_by_id (
        'Polygon_2', 
        $id
    ), 
    'depth_2'
)

Let's suppose you have two layers:

  1. Polygon_1 with attribute depth_1 (on this layer, you open the field calculator)
  2. Polygon_2 with attribute depth_2 (this is the value you want to get in the other layer)

You can get the value from one layer to the other one if they have a common field that they can be connected with - here: the same id. Of course, it works also with other conditions. But since in your question you have not stated how the features of the two layers should be related, I suppose for demonstration purpose that they are connected by a commond id. If not, change the expression accordingly.

With the expression above, for each feature on Polygon_1, you get the according value (of the feature with the same id) from Polygon_2, namely value of field depth_2. You can use this to copy values from Polygon_2 to Polygon_1. If you add "depth_1" - before the above expression, you get the difference depth_1 - depth_2 (what you want), see the screenshot:

enter image description here

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