How can I modify an existing DEM using a polygon? I have a DEM raster with terrain varying between 355 and 365 meters. I have a polygon I've drawn over the area. Ideally, I would like to uniformly reduce the DEM by 1 meter from its current height within the polygon (so new values will range from 354 to 364 meters).
I found this previous question and answer, however the workflow seems to have changed as I'm having trouble following the answer in QGIS 3.4. Also, that answer addresses how to alter the DEM to have a new absolute height based on the elevation values of the polygon, but now how to create new height values relative to the old height values. The reason I want to do this is because the terrain in question is on a slope, so I cannot simply assign a single elevation value to the entire area bounded by the polygon.