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Working under QGIS 3.16.4 and using the Modeler, I would like to create buffers for a polygons layer.

The polygons layer includes the buffer size as an attribute (e.g. "BUFFER_M") resulting from the field calculator (a previous step in the model).

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Although I specified the polygons layer as the Input layer (Using algorithm output), there is no fields available (NULL) in the Expression Dialog.

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I tried to fix the Distance with the expected BUFFER_M field as following: Pre-calculated Value > "BUFFER_M" in the Expression Dialog with no success.

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All other steps in the model worked well but the buffers layer was generated with a Distance default value of 10,00 m.

Any ideas on how to specify the buffer size (Distance parameter) based on a field attribute of the input layer (vector layer)?

NB: the same problem when done manually as well (generated buffer of 10,00 m), however in this case the "BUFFER_M" field was available in the Expression Dialog.

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  • If you want to use an attribute from another layer, how should QGIS know which buffer to use for which feature on your current layer?
    – Erik
    Mar 24, 2021 at 12:58
  • Not sure to understand. The buffer size attribute is already in the table of the layer containing each features on which I would apply the buffering (with values lines per lines). Would you suppose I need to specify, e.g. : 'buffer_size by id' in the Expression Dialog?
    – Jérémie Beghin
    Mar 24, 2021 at 13:23
  • You wrote "based on a field attribute from another vector layer" - which implies that you want to buffer layer A by values from layer B.
    – Erik
    Mar 24, 2021 at 14:22
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    I recognise it was unprecise, I should have said "from a field of the input layer". Edited now.
    – Jérémie Beghin
    Mar 24, 2021 at 14:30
  • See here: gis.stackexchange.com/a/420165/88814
    – Babel
    Jan 4, 2022 at 16:38

1 Answer 1

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Try the SAGA's Module 'Variable distance buffer'.

In the Buffer Distance field, please write the name of your field, that defines the buffer size.

result

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