Remove unwanted vertices from a straight line

Question

I have a MultiLine String that is straight but when I highlight vertices, I notice that it has 10 vertices - some vertices also overlap. When I covert this multipart line to singlepart, the line still has 10 vertices.

I would like to simplify this line (which has one single id) to have only one a start and end point.

I have tried dissolving the line, fix geometry and simplify and can't seem to remove the unwanted vertices

Answer 1

You can use the following expression to eliminate all vertices from a line that deviate for a maximum distance from the direct line connecting the start- and end-point of a line. Use Geoemtry Generator or Geometry by expression (see here for details).

• The expression evaluates for each vertex of the initial line, except for start- and end-point, if a buffer of distance x (here: 50 on line 20, change this value to fit your needs) intersects the direct line connecting start- and end-point.

• If so, the output is an empty element, otherwise the output is the respective vertex as a point geometry.

• So for each vertex, these outputs are collected in an array and then the array is filtered to retain only elements > 0. You get an array than only contains points.

• These ponints represent the vertices further away from the line connecting start- and end-point than the distance you defined.

• Add start- and end-point of the initial line at the beginning and end of the array and convert the array to a line and here you are!

make_line (
    array (
        start_point ($geometry),
        collect_geometries (
            array_filter (
                array_foreach (
                    generate_series (2, num_points( $geometry)-1),
                    case
                    when        
                        intersects (
                            make_line (
                                start_point ($geometry), 
                                end_point ($geometry)
                            ),
                            buffer (
                                point_n (
                                    $geometry, 
                                    @element
                                ), 
                                1  -- change this distance
                            )
                        ) = 0
                    then point_n ($geometry, @element)
                    end
                ),
                @element>0
            )
        ), 
        end_point ($geometry)
    )
)

Black line with initial vertices (red dots) as well as the vertices of the line created by the expression above in blue (adding a nodes_to_points() function to the expression) - as you can see, just 2 vertices (start- and end-point) remain: