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I have a DEM and a line vector shapefile (also available in PostGIS) and I need to calculate the mean slope of every line. Direction of the slope (positive or negative) is not important.
That means I should assign the DEM raster values of the start and end vertex to two new attributes (e.g. alti_a and alti_b). Then it would be possible to use field calculator and calculate the slope.
How can I do this? Are there alternative solutions?
Assuming you have DEM height (z) at both ends as "z1", "z2" fields respectively in your line layer, try this expression in the Field Calculator.
degrees(atan2(abs("z2"-"z1"), sqrt(($x_at(-1)-$x_at(0))^2 + ($y_at(-1)-$y_at(0))^2)))
Grid values to Point tool, z values are stored in point layer. Join line and points together. To be honest I usually manually copy and paste z-values as I do not have too many lines.
abs(), either will do. It is still better sticking to your own rule, just to avoid potential confusion (for future updates).
To get the elevation / z values from the DEM for (all) the vertices of your line geometries, you can use the Drape (set Z value from raster) QGIS algorithm.
You can then use Extract specific vertices to get to the start (0) and end (-1) points of the lines.
(or you can extract the start and end points first and then get the z values from the DEM)
In your calculations you can then use the z(geom) function to get the z (elevation) values from the geometries.
