Creating points along the line on either side of an intersection point

Question

I have a center line and an intersection point. I want to create points on either side of this intersection point along the center line with an interval 50 meters(can vary) starting from the intersection point. I tried "Points along geometry" which didn't give desired result in QGIS.

When I split the line proceed with "points along geometry" I get the points which are not at 50m interval from the intersection. In the image below the distance between the highlighted points is not 50m

I want something like creating points on either side with negative chainages on one side and positive on the other, so that I can clearly identify the points after exporting them to csv.

Answer 1

The idea

Use the following expression on the line layer with Geometry by expression or Geometry generator (see here for details) and make setting accordingly:

• Replace intersection (line 3) with the name of the point layer containing the intersections.
• Replace the distance of the interval for the points to be created in line 6.

Possible variants:

• If creating actual points using Geometry by expression, you get multipart geometries. Convert them to single points using Menu Vector / Geomtry tools / Multipart to single parts.
• This expression creates points along the whole length of the line. To create only a limited no. of points, see at the bottom.

The expression to use:

with_variable (
    'inter',
    'intersection',
    with_variable (
        'distance',
        100,
        collect_geometries (
            array_foreach (
                generate_series (
                    floor(
                        line_locate_point( 
                            $geometry,
                            overlay_nearest (
                                @inter,
                                $geometry
                            )[0]
                        )/-@distance
                    )+1,
                    floor (
                        (length ($geometry)-
                        line_locate_point( 
                            $geometry,
                            overlay_nearest (
                                @inter,
                                $geometry
                            )[0]
                        ))/@distance
                    )
                ),
                line_interpolate_point( 
                    $geometry, 
                    line_locate_point( 
                        $geometry,
                        overlay_nearest (
                            @inter,
                            $geometry
                        )[0]
                    )+@distance*@element
                )
            )
        )
    )
)

Expression used with Geomtry generator, creating points on both sided of the blue intersection points at a regular distance of 50 m along the line:

Create only a limited no. of points

To create only a limited no. of points, change the above expression. Replace the generate_series () part (lines 9 to 29) with this expression: generate_series (-5,3), where 5 (preceded by a minus!) is the no. of points between the intersection and the start-point of the line, 3 is the no. of points from the intersection towards the line's end-point.

Be aware: if one of the lines is too short to accommodate all points you want to create, no points at all will be created. In this case, decrease the numbers.