Connecting datapoints of same individual on same date with line in QGIS

Question

I am currently doing fieldwork on little owls. Part of this is noting down on a map where I hear their sounds coming from. These points I later transfer to QGIS (image 1).

Sometimes the owl moves while I'm recording data, so I have two different locations for the same individual (image 2).

I would like to connect these different locations with a line in QGIS. The lines need to be grouped per date, so the line only links the locations of the individual together of one certain date (so not all the locations the individual was heard from on different dates). I hope this makes sense.

How could I do this in QGIS? I've tried points to path, but this connects all the locations of the individual, so not sorted per date.

Answer 1

(QGIS 3.22.3) In the Points to path tool, you can use an expression in the Path group expression option to group your data.

Click the button on the right of the field to open the expression editor.

Enter this expression. "date" and "indiv" are field names from your point layer, || is the concatenate operator. Be careful with double " and single ' quotes. Double quotes are for field names, single quotes are for strings.

"date" || '_' || "indiv"

It concatenates the date, an underscore, and the individual's ID. The result of the expression would look something like 2022-02-21_1 (and so on for each combination of date and indiv).

You won't see this calculated anywhere (except in the preview in the lower left of the expression editor). But the Paths to points tool will use it to group your points.

The paths are then generated per group.

It just so happens than owl 3 in my randomly generated data set only had a single observation per date and therefore didn't 'move'.

---

Variation:

For a more explicit version of the same concept (which will work on QGIS versions before the Path group expression option was added), you can use Field Calculator (with the same expression "date" || '_' || "indiv") to generate the groups in a new Text field (called group).

And then select this field in the Group field of the Points to path tool

Answer 2

To visualise lines between grouped points you can use Geometry Generator on a layer symbol in the point symbology.

Expression:

make_line(
    array_agg(
        expression:=$geometry, 
        group_by:=date||heardfrom||indiv,
        order_by:=heardfrom
    )
)

(I added the labels and colours to better visualise the example).

Note:

It is not a perfect solution as outlined in this question, but it will update dynamically as the point layer is edited.