maskingqgisraster-calculator
How do I change the values of raster cells which are within a polygon, leaving all cell values outside the polygon unchanged?
I would like to subtract 1.5m from each raster cell value within the polygon, rather than make them all the same value.
I don't know how to use GRASS so I'm hoping there is a way to do it using the tools within the QGIS GUI, raster calculator, or processing toolbox. I'm using QGIS 2.0.
Because the Raster Calculator is a spatial analyst tool, you can utilize the Mask environment.
From there, you can use a variety of commands to perform the reclassification: common ones include Con, Pick, Is Null and Set Null, based on your needs.
To check if a specific spatial analyst tool honors the Mask environment, simply scroll down to the bottom of the help page and check the available environment settings.
Found that I could create a polygon mask of desired shape using Layer>Create Layer>New Shapefile Layer... Including an integer field to be filled with "1", then converting this layer into a raster using Raster>Conversion>Rasterise, selecting your shapefile and choosing the integer field as the attribute field - I didn't alter the default pixel size given of 3000x3000 and saved in GeoTIFF format.
Using my new Raster I went to the Raster Calculator and did as described in: How to change raster cell values within a polygon using QGIS GUI? with the expression yourdem@1 - ((poltoras@1 = 1) * x ) With x being the desired reduction in height and being able to increase height within the polygon similarly. Once again saving the output in .tif
From there I converted my original topography.asc file into topography.tif using Raster>Conversion>Translate to choosing the original ASCII file and choosing the output format to be GeoTIFF. From there I merged the topography and altered elevation .tif files and then converted the resulting file back into .asc using the process described above.
Best Answer
First, you need to convert your polygon to a raster. Create an integer field filled with "1" for the purpose of the conversion. (raster > conversion > rasterize)
Then, you can use the raster calculator to substract 1.5 where your polygon exists.