Extract Single (or small number) Contour from DTM data in QGIS

Question

I understand that Contour data can be extracted at particular intervals fairly easily, but have been recently mapping a number of Flood Water Design levels which are denoted to the nearest 0.001m. It is quite time and computationally hungry to generate all these contours, only to then delete pretty much all of them out to get the particular level contours I am looking for.

Is there a way, in QGIS, of generating only a single (or single elevation if not continuous) contour at a specified value from .tif based Lidar DTM data?

Answer 1

You can use r.contour:

Produces a vector map of specified contours from a raster map.

Answer 2

QGIS is using this GDAL utility https://gdal.org/programs/gdal_contour.html and it has an option to create contours not for intervals but for a fixed set of levels

-fl

Name one or more “fixed levels” to extract.

You can add the fixed levels in the QGIS user interface as "additional command-line parameters"

The generated gdal_dem command will look like

gdal_contour -b 1 -a ELEV -i 10.0 -f "GPKG" -fl 10 C:\data\test.tif C:/Temp/processing_qWriLo/7f1037ec5dde48eab15b9643b7a4d845/OUTPUT.gpkg

QGIS seems to keep the interval parameter -i but it is skipped in the processing and contour lines are created only for the levels which are listed with -fl.