I have a raster elevation map that has two tiers of known elevation, and I am trying to produce a raster that will smoothly contour the area between the two tiers with a gradient corresponding to the slopes between the known elevations. Basically, the interior boundary of the raster (and everything inside it) has an elevation of, say, 600m, and the outer boundary is at 624m. The black areas are zero values that will be snipped away and are irrelevant. I would like to generate contours between these two elevation values in a raster format to visualize the slopes between the two elevations. The final raster will be subtracted from a DEM to visualize an excavation accurately.
Please see the attached image of the raster example to see what I am working with. I am flexible with my methodology but I am trying to keep my workflow 100% within QGIS as I am sure there must be a solution here. I have been looking at many of the functions included in GRASS but have not been successful in getting them to generate contours based on this data.