Adjusting flow direction of river based on DEM in QGIS

Question

I have a huge vector layer as a line, that represents the river. The problem is that the flow direction is sometimes wrong (goes uphill). But I can't correct that manually, cause the layer is too big.

I also have a DEM for the entire area. So is it a way to reverse only the lines that go uphill? I mean something like selecting the line that the end_point is at a higher altitude than the start_point, and flipping the selection?

Answer 1

Solution 1

1. Use select by expression with this expression where DEM is the name of your raster layer (digital elevation model):

   raster_value ('DEM', 1, start_point ($geometry)) <
   raster_value ('DEM', 1, end_point ($geometry))
   

2. Then reverse line direction for selected features (see screenshot below).

Solution 2

You could also do it directly in one step with Geometry by expression, creating a new layer:

if (
    raster_value( 'DEM', 1, start_point ($geometry)) <
    raster_value( 'DEM', 1, end_point ($geometry)),
    reverse( $geometry),
    $geometry
)

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The line at the botten right will be selected: