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I've generated some routes with the Network Analysis: Shortest Path (Layer to Point) from the Processing Toolbox (PT). Route files are GPKG.

Now, I am trying to get the tapered buffer, also from the PT, of these routes. In most cases it works fine, but not always.

I received the following error message:

Feature could not be written to Buffered_a8dd9771_152d_4105_b1f4_b16ca36f413d: 
Could not add feature with geometry type GeometryCollection to layer of type MultiPolygon

The problem is only with the tapered buffer. No problem with the normal buffer tool.

I can imagine that the problem is the geometry of the routes, so unsuccessfully I tried:

  • to fix the geometries: sometimes all are fixed and other times some are lost
  • to change the geometry type of the paths from "automatic" to "linestring"
  • to convert to single parts...

The problem only happens when I use an expression for the end width. It works fine with a number but it's not a problem of the expression: it works fine with other routes.

I finally got it: the problem was...me. I generated a few hundred data tables through the processing modeler and later caused a problem with the table names. I needed to add some fields manually and... error: some of the column names were similar to expected but not the same. The bad column name causes a processing error with the tapered buffer expression. I thought based on the error message that the problem was the geometry but it was the field name.

(The clue was here: Feature could not be written to Buffered_a8dd97...)

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    Please provide (faulty) example data and your exact buffer settings.
    – Erik
    Commented Aug 9, 2022 at 11:21
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    It seems that the buffer function creates multipolygons for most geometries but for some geometries it has to create geometry collections which probably contain a combination of polygons and lines. For fixing the issue the output layer should be created as a generic geometry type "geometry" in order to accept any kind of geometries. Unfortunately I do not know how to do that.
    – user30184
    Commented Aug 9, 2022 at 12:19
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    @user30184 maybe via a virtual layer? select tapered_buffer(geometry,0,100,16) from l
    – JGH
    Commented Aug 9, 2022 at 14:11
  • @JHG. Thxs. Is not a virtual layer. Where can I use the expression? I can't run it in the phyton console.
    – david
    Commented Aug 10, 2022 at 7:25

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