Representing day/night routes in QGIS

Question

I'm working with QGIS 3.22.11-Białowieża and I've got a point layer that represents the migration route of a bird species. Each point has the following attributes:

• trip that identifies a single trip (all points of a certain individual and route -northwards or southwards- have the same value for that field)
• time_stamp that holds date for that position
• day_night that indicates whether the position was obtained during day or night time.

Using the Point to Path algorithm I can easily represent each migration route, using the trip attribute as grouping expression and the time_stamp as the order expression. But I need to represent in different colours the night and day stretches of each route, as the points in the image below, and the point-to-path algorithm doesn't seem to be able to hold that variable (only the trip id and the begin and end date).

Data for this example can be downloaded here.

I've found this thread with some hints, but my knowledge of Python is still too limited. Is there a way or workaround to do that representation?

Answer 1

Divide the line in segments and assign each segment the daynight value of the closest point:

1. Explode lines

2. Add an attribute with field calculator: overlay_touches ('foo_points', daynight)[0]

3. Assign different colors based on this attribute with Categorized layer styling.

Even easier, I introduced the expression directly in the Categorized styling dialog, thus doing step 2 and 3 in a single step:

If you want to filter to catch only points with the same trip id (what seems not necessary, as long as you don't have points belonging to different trips in exactly the same spot, what seems unlikely), it's a bit more complicated. Unfortunately, filtering inside overlay functions in QGIS 3.28 still does not work as described here. But you can use this expression instead:

array_sort (
    array_foreach (
        overlay_touches ('foo_points', $id),
        if (
            attribute (get_feature_by_id ('foo_points',@element), 'trip') = trip,
            attribute (get_feature_by_id ('foo_points', @element), 'daynight'),
            ''
        )
    ),0
)[0]