How to calculate the azimuth of an offset point/line?

Question

After receiving and answer to this question, I am stuck with another problem.

For context, my end goal is to assess windward/lee side of a forested edges (line segments) in respect to a dominant wind direction. Following the answer in the linked question, I have created an offset point on the inside edge of each line segment. I now am trying to evaluate these line segments based on the azimuth of the offset (segment midpoint to offset midpoint).

How can I reference the offset point in an expression in a field calculation on the original line segment?

---

How do I reference offset_point if it is in another layer?

If it is another feature in the same layer?

psuedocode: 
offset_azimuth = degrees(azimuth(segment_midpoint, offset_point))
if(offset_azimuth = (windDir +/- 45), "Lee", "Windward")

Answer 1

If I understood your question right, the azimuth you're looking for is the azimuth of the small red lines in the original answer. I created this line using the following expression on the layer with the resulting points (those inside the polygons). It includes the overlay_nearest function, available since QGIS 3.16. The first argument is the reference to a layer - in my case named 'exploded' - rename it for the name of the layer you use:

shortest_line (
    centroid (
        array_first( 
            overlay_nearest( 
                'exploded', 
                $geometry
            )
        )
    ), 
    $geometry
)

You can get the azimuth of this line using the function azimuth(point_a,point_b), where point_a is the start- and point_b the endpoint of the line created above. To avoid repeating the whole expression from above, add the function with_variable(name,value,expression) to define the line as a custom variable @line that you than can use with start_point (@line) and end_point (@line).

Finally, the azimuth function retunrns the azimuth in radians - you probably want to convert it to degrees, so convert the whole expression with the function degrees(radians).

The final expression looks like this:

degrees( 
    with_variable( 
        'line',
        shortest_line (
            centroid (
                array_first( 
                    overlay_nearest( 
                        'exploded', 
                        $geometry
                    )
                )
            ), 
            $geometry
        ),
        azimuth (
            end_point (@line),
            start_point (@line)
        )
    )
)

Here is how the degree is calculated: I used the expression from above to define a label (I added a round function to round it to one decimal):