Interpolate Coarse Raster Contours

Question

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.

Answer 1

You may want to consider using the Interpolation plugin in conjunction with a vector layer.

Create a line vector layer in which you have two shapes similar to the rasters in your question, and assign them the appropriate elevation values.

In QGIS 2.18:

Open the Interpolation plugin and enter the following:

1. Input
   1. Vector layers: your_layer
   2. Interpolation attribute: elevation
   3. Click Add
   4. Select Type: Break lines
2. Output
   1. Select Triangular interpolation (TIN)
   2. Set your columns and rows as you like, as well as the extent.
   3. Output file: your_output.tiff
3. Click OK, then load your newly created raster:

---

In QGIS 3.0:

TIN interpolation is now built-in, and can be found under Interpolation in the Processing Toolbox. The same settings from above can be successfully applied.

You should be able to use this output with your DEM raster. If you need more than a two-point linear slope, simply add other intermediate lines in your vector layer before running the interpolation.