I want to elevate a certain area of terrain by 126 meters. I've created a polygonal shapefile layer with a decimal field titled "elevation" set to 126. My plan was to rasterize the polygon using GDAL Rasterize, set the null areas to 0 using GRASS r.null, and then add the result to the original elevation raster. When I used GDAL Rasterize, the resulting raster was transparent, I assume because it was constituted completely by null values.
How can I turn my polygon into something that I can add to the original elevation raster?