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Background:

I need to create lines from points where the point attributes are transferred to the line segments.

The screenshot below shows two point tracks. Each track's point has a unique id and an additional attribute, in this case elevation.

enter image description here

Here's a screenshot showing the point layer's attribute table:

enter image description here

Below is a screen shot showing the desired line output. Notice that each line segment now contains both of the elevation values from the two points that comprised that line segment.

enter image description here

The resulting line attribute table includes three new fields: line_segment, elevation_begin, and elevation_end.

enter image description here

Question:

I don't know how to create new lines that contain attribute values from the source points. None of the "point to line" tools or plugins offer this functionality. There used to be a plugin named "Points2Paths" that could do this, but it has been removed from the repository.

I suppose that Geometry by Expression might offer a solution, but I don't know where to begin...

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  • Geometry by Expression is going to struggle because I don't think you can use it to create new attributes (from old ones). You could possibly do this separately with expressions on the attribute table. Also you would probably need to create a helper attribute combining the track and id, as while you can use get_feature to return a feature based on an attribute, it only works for one attribute (and not two). So you might be looking at 4-5 steps. Oct 3, 2023 at 23:27

1 Answer 1

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I imagine you're right that Geometry by Expression could be used but my brain reflexively thinks of an SQL solution:

Layer|Create Layer|New Virtual Layer...

enter image description here use the following for the SQL, substituting "point_track" with the name of your point layer

SELECT p1.track,p1.id AS line_segment,p1.elevation AS elevation_begin,p2.elevation AS elevation_end, make_line(p1.geometry,p2.geometry) AS geom 
FROM point_track p1 
JOIN point_track p2 ON p2.track=p1.track AND p2.id=p1.id+1

enter image description here

Edit: As Stu pointed out (and I neglected to mention) this query assumes sequential integer type id numbers. I think its a quirk of the SQLite engine that it also works when the numbers are in a text field. Rather than complicating the SQL, if you do happen to have non sequential ids (or text ids like a, b, c..) the simplest approach might be to use the Processing Toolbox "Add autoincrement field":

  • Group values by "track".
  • If you have numeric ids in a text field use Sort expression to_int("id")
  • otherwise just sort by "id"

Then adjust the query to use the newly generated sequential id field: enter image description here

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    Or use Execute SQL (Vector General, Processing Toolbox) rather than a Virtual Layer if you don't need the layer to update on the fly. Oct 3, 2023 at 23:29
  • @Tom Brennan - thanks, I didn't know about that one,
    – M Bain
    Oct 4, 2023 at 0:08
  • Thanks to both Bain and Brennan - both of their solutions worked just fine. This point-to-line approach would make a great addition to the Plugins library - if only I knew how to do that! Also, here are a couple of items based on my testing of this solution: 1) The "track" and "id" fields will work either as integer or string types, and 2) The "id" field values must be sequential without gaps (i.e. 1, 2, 3... not 2, 4, 6...)
    – Stu Smith
    Oct 5, 2023 at 22:30
  • Also, I'm pleased to report that the resulting lines are in increasing "id" direction - just as if they were correctly digitized in an ascending, from-lower-to-higher order.
    – Stu Smith
    Oct 7, 2023 at 1:46

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