Generating points along line with specifying the origin of point generation in QGIS

Question

I have been trying to generate points along a line with a distance of 500m. We can do this in several ways using tools like "Points along Geometry". However, my problem is that I do not know how to generate points in a specific direction along the line.

I tried using "Points along Geometry", but there is no option to specify the origin. Below you can see what I get using "Points along Geometry".

The problem is that the generation is going down to up and I want it up to down. Is it possible to choose the origin of the line to start generating points?

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Answer 1

The tool Points along Geometry follows line direction and thus begins where the line has the first vertex (start point). To start from the other end (the last vertex, end point), reverse the line direction:

Use Menu Processing > Toolbox > Reverse line direction, then create the points on the resulting line.

Answer 2

You can create a Virtual Layer using functions ST_Line_Interpolate_Equidistant_Points and ST_Reverse

Replace road with the name of your layer.

To get the point starting from line starts:

SELECT row_number() over() as id, ST_Line_Interpolate_Equidistant_Points(geometry, 500.0) as geometry
FROM road

To start from the line ends, reverse it:

SELECT row_number() over() as id, ST_Line_Interpolate_Equidistant_Points(St_Reverse(geometry), 500.0) as geometry
FROM road

You will get a multipoint output. You can convert them to single parts with "Multipart To Singleparts"

Answer 3

Use the following expression to create points at a regular interval along a line. Set the interval distance in line 1 (in my example: 80). Line 2 is for the option you're looking for, "specify the origin": change 0 to 1 to start the creation of the points from the other side (starting from the line's end point). For this, I used the function reverse(), together with an if() clause (line 7).

Use the expression with Geometry Generator (for visual purpose/testing the result in realtime) or Geoemtry by Expression (for actual geometries) - see here for details.

By the way: if you want to add a start offset, in line 6 simply replace 0 by the distance you want the first point to offset. For end offset, in the same line add -100 (or whatever distance) directly after length($geometry) - before the comma.

with_variable ('interval',80,  -- distance between points
with_variable ('direction',1,  -- set to 1 to reverse line direction (start creating points from the line's opposite end)
collect_geometries (
    array_foreach(
        generate_series (0, length($geometry),@interval),
        line_interpolate_point (
            if(@direction=0,$geometry,reverse($geometry)), 
            @element)
        )
    )
))

The expression, used here with Geometry Generator; lines start at the left, but points are being created starting from the right because in line 2, tha variable @direction is set to 1: