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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?

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Maybe you might forgot to set attribute for your polygon features (in your case 126), which GDAL Rasterize needs for processing. Example:

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GDAL rasterize automatically set NULLs to 0 (Zero) if not set otherwise.

GRASS v.to.rast.value makes the same job well, even without a need to attribute the polygon features. Here, you set the desired value directly in the dialog. Then (as you suggest) r.null can be applied.

In both cases, resolution (and output size) needs to be set correctly (exactly as target raster).

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