Calculating route kilometer (min and max) for intersection

Question

How would you solve the problem:

1. Points with km values every kilometer
2. A route (f.e. railroad) als polyline
3. An area where you would like to know at what kilometers it intersects with the route (railroad km_min and km_max)

Answer 1

You can use the field calculator on your area layer with the following expression:

Where:

• route: is your line layer
• area: is your polygon layer
• points: is your points layer (with a field called distance - I use meters - to use km you will have to convert in the expression)

-- make variable from array of all distances of intersecting points
with_variable(
    'distarray',
    overlay_intersects(
        'points', 
        "distance"
    )
    ,
    -- make variable of the geometry of the point with minimum distance
    with_variable(
        'start',
        array_filter(
            overlay_intersects(
                'points', 
                array("distance", $geometry)
            ),
            @element[0] = array_min(@distarray)
        )[0][1],
        
        -- make variable of the geometry of the point with maximum distance
        with_variable(
            'end',
            array_filter(
                overlay_intersects(
                    'points', 
                    array("distance", $geometry)
                ),
                @element[0] = array_max(@distarray)
            )[0][1],
            
            -- make variable of the route intersecting the area polygon
            with_variable(
                'intersectline',
                intersection($geometry, aggregate('route', 'collect', $geometry)),
            
                -- get length of line between the start of the intersecting route and the first point and subtract it from the minimum point distance
                round(array_min(@distarray) - line_locate_point(@intersectline, @start), 1) 
                
                -- concatenate the two resulting values into a string, separated by a comma
                -- of course, the two values can be calculated separately into two different fields if you prefer
                || ', ' || 
                
                -- get length of line between the end of the intersecting route and the last point and add it to the maximum point distance
                -- the line is reversed so that `line_locate_point` starts at the other end of the intersecting route
                round(array_max(@distarray) + line_locate_point(reverse(@intersectline), @end), 1)
            
            )
        )
    )
)

The blue labels in the polygon show the length of lines I drew as a sanity check between the ends of the intersecting route and the first/last points.

Note: this will have limitations if the route is very twisty and intersects the area more than once.

Reference

QGIS expression - overlay function, using overlay function as filter