Finding nearest line feature from linear layer in QGIS

Question

I have two linear layers like you see below:

I would like to select just the nearest black lines to the red ones, which represent the other layer.

I found similar solutions, but they refer to the point layer instead:

Find nearest line feature from point in QGIS

Is there some way of making a selection of just these lines, which lie in the direct vicinity of the red line (another layer)?

UPDATE I:

The answer below brought me to the following approach:

the formula works, but I guess something is wrong with the Join by nearest option. I've set a unique id for both layers, one of them had already fid as the unique one. When the formula is applied, some parts of the layer are selected regardless of their vicinity to the other layer.

How should I do the Join by nearest option with respect to this situation?

Answer 1

You need a unique id field in your black line layer, mine is called id.

Join (by nearest) this id to the centerline so each centerline get the attribute of the closest black line. First split the centerline into short segments to be sure each nearest black line is found.

In the output you will have only the black ids closest to the centerline, the ids you want to select.

My output layer is called 'joined'. Select by expression on your black line layer with the expression:

array_contains(array:=
aggregate(layer:='joined', aggregate:='array_agg', expression:="id"), value:="id")

Answer 2

Use QGIS expressions with function overlay_nerarest(), available since QGIS 3.16, to identify the nearest line from another layer (here called layer_2). On the first layer (with the red line in your screenshot), run Geometry by expression with the following expression to create a layer containing the closest line:

overlay_nearest('layer_2', $geometry)[0]

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Like this, you already have a layer with the lines you want. If you indeed want to have a selection on your initial layer_2, proceed as follows: On layer layer_2 use Select by Expression with this expression:

overlay_within('Modified geometry')

Answer 3

As you made clear in your comment, you're using QGIS 3.12, so you can't use the very handy overlay_nearest() function, introduced in QGIS 3.16. So for older versions (tested with QGIS 3.12), you have to use a more complicated approach (that sayed: I heavily recommend updating to the current version!). Use this expression, replacing line2 with the name of your line layer from where you want to find the closest line feature:

array_filter (
     array_foreach(
        aggregate ('line2', 'array_agg', $geometry),
        if (
            intersects(
                buffer (@element, 0.1), 
                closest_point (
                    aggregate ('line2', 'collect', $geometry),
                    $geometry
                )
            ),
            @element,
            NULL
        )
    ),
    @element is not NULL
) [0]

Screenshot from QGIS 3.12: red line=layer line1, black lines= layer line2; yellow highlighted line is created with the expression from above, applied on line1 (red line) to find the closest one of the black lines:

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If you need a selection on the layer with the black lines, use the expression from above with Geometry by expression to create the line as a separate layer. Than go the the initial black line layer, use Select by expression with this expression, where Modified the the name of the layer created with Geometry by expression:

intersects ($geometry, aggregate ('Modified', 'collect', $geometry))