I'm trying to create a simple labeled contour line layer for a topographical map in QGIS. I've created a new layer for the contours using "Raster > Extraction > Contours" on a raster DEM GeoTIFF. The original data is in meters, and I want the contours to be at reasonable intervals in feet, so I've taken the newly created vector layer and added a field which converts all the data points in meters to feet.
Great, my labels can now show feet… but the contours have already been created at intervals based on meters, which are now sort of arbitrary. Adding labels at this point yields results like "5,668 ft, 5,889 ft" etc.
At this point, I don't know how to redefine the placement of contours in this layer, or to create a new contour layer that references the newly created field with converted units. How can I tell QGIS that I want this layer to draw contours from the new "ELEV-FT" field I've created w/ converted elevations, and how can I modify the "offset from zero" parameter that determines the starting elevation of the contour intervals?
To get even more fundamental: How in QGIS can I create contours at regular, whole number intervals in feet from a DEM raster source that has a base elevation of 1,643.531982 meters?
Best Answer
If you just want to play around and try different settings before creating actual countours, use the layer rendering style
Contours, available since QGIS 3.14 - see also the documentation. This creates contours as a style, not actual geometries. You can make different settings and the contours will change in real time (as long as your machine has enough power to render it in real time for the selected extent - otherwise you have to wait a bit or zoom in).Like this you can easily find out what fits your needs and when satisfied use the interval to create actual contours (vector layer).