Atlas with grouped or connected lines as coverage layer (QGIS)

Question

I have a line layer containing sections of paths. Sections with the same ID (WID) form a hiking trail. Many of these trails are circular routes, which often have access/exit paths starting from a parking lot. I want to create a sort of "route book" using the QGIS Atlas, with the hiking trails (not the individual sections) serving as coverage layers. On each atlas page, I would like to display the length of the main route and the length of the access/exit paths or sub-routes.

I can create the atlas features for the hiking trails using a virtual layer and the ST_Union function (group by WID), but then I cannot access the access/exit paths or sub-routes anymore. How can I display the length of the main route and the sub-routes for each complete hiking trail on each atlas page?

The Linelayer with selection:

The Atlas-Page:

Answer 1

Assuming your virtual layer uses a query like

SELECT WID, ST_UNION(geometry) AS geometry
FROM
Routen
GROUP BY
WID

i.e. it has the UNIONed geometry and the WID, I would suggest using the virtual layer geometry just for your coverage layer extents, but use the WID in the virtual layer data to lookup the lengths from the original data and return the result in a textbox in your print layout.

Example data:

Virtual layer:

Example expression in textbox to look up main and subroute lengths separately, joined by WID from virtual layer (=atlas coverage layer)

array_to_string(
    aggregate('Routen',
              'array_agg',
              "Category"||': '||"length"||' m',
              "WID"=attribute(@atlas_feature,'WID')
             ),
'\n ')

You can of course modify the expression in so many ways:

• to only show length instead of "Category"||': '||"length"||' m'
• add an extra condition to the filter such as AND "Category" = 'Subroute' so that you can have a text box only showing subroute lengths
• use sum instead of array_agg as the aggregator
• get total lengths by summing up lengths without the category filter...

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Result: