Treating a feature as both a single line (for length) and a multi line (to colour each segment)

Question

I am tracking progress made each day along a path, so that I can show in QGIS each day's progress in a different colour with a label showing the day number and distance covered.

However in this case each line segment is a separate feature (required in order to label and colour the line segments differently using categories) so I am not sure how to apply a global label that contains the total distance of all the segments combined.

It is also difficult because using the LineString geometry and having a separate feature for each segment means they don't exactly start at the same point as the previous segment finished, depending on how accurate my mouse clicks are or how willing I am to copy and paste coordinates when editing the vertices.

Is there a way to record this geometry such that it can be treated as a single feature with only one long line, but also to colour and label the individual segments differently, based on an arbitrary value like the day the work was performed?

I do not mind having to change the geometry of the layer to make this work, but I don't want to have to do any double data entry or otherwise synchronise between two copies of the data each time I add a new segment.

I looked at the MultiLine geometry but this looks like I can have unconnected line segments all sharing a single label which is the opposite of what I'm after - I need only connected line segments each of which has a different label, but where I can provide a global label (and length measurement) to the entire line as well.

Is there any way to achieve this?

Answer 1

For the problem that each segment don't exactly start at the same point as the previous segment finished that is easily solved by using snapping.

Then for getting the total length while keeping each part a different color I suggest using a virtual layer that will union all geometry and give you the total length. As virtual layer are dynamic (they are based on an SQL query) it will recalculate each time you modify your original line layer (so you do have two layer but without having to do any double data entry or otherwise synchronise between two copies of the data each time you add a new segment)

The SQL query for the virtual layer could be something like this :

Select
    Count(NAME)              as 'NUMBER_OF_SEGMENT'
  , sum(st_length(geometry)) as 'TOTAL_DISTANCE'
  , st_union(geometry)       as geometry
from
    LINE_LAYER

(Change NAME by any attribute column from your line layer and LINE_LAYER by the actual name of your line layer)

Below you can see the result with the colored individual segment and on top the black virtual layer

Answer 2

An option that requires no new layers is to use a Geometry Generator expression to generate the total line for labelling, as well as its length.

To set this up, under Labels, enable Rule-based labeling, with two labels:

Global label

The first label is the Global label, which uses collect to aggregate all of the individual features into one for measuring the length, via the following formula for the label:

'Total length: ' || round(length(collect( $geometry, group_by:=overlay_nearest(@layer, $geometry))),2)

Under Placement, you also need to enable Geometry Generator:

This uses a cut down version of the formula above:

collect(
$geometry,
group_by:=overlay_nearest(@layer, $geometry))

This will create one label for each feature (4 in the example). This can be restricted to 1 label total by using a generic filter, which labels the feature with the lowest ID only:

to_string(@id) = 
array_min(
    string_to_array(
        aggregate(
            @layer,
            aggregate:='concatenate',
            expression:=to_string(@id),
            concatenator:=','),
        delimiter:=',')
    )

Individual label

The second label is the Individual label for each segment, which just uses the formula:

'Length: ' || round($length,2)

Final result is as follows:

As mentioned elsewhere, use Snapping to ensure features start/finish at the same points.