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I am working with two raster files. The first is a DEM and the second an elevation-correction file. Both files display the same geographic region. My problem is that the cell sizes of both files aren't equal. The DEM contains 5 million pixels (values), whereas the second file contains only 30 which means the cell size is much broader. My wish is to subtract the second file from the DEM. I tried using Raster Calculator but the result is not satisfying since this destroys the whole DEM.

My approach is the following: I have to use an expression within the Raster Calculator that tells the software to subtract every single DEM-cell from the corresponding value of my correction file. But unfortunately I haven't figured out how to do this... Does anyone of you have an idea how to do this?

I am using ArcGIS 10.5 and QGIS 3.4.

Vince
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MAT
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    Use environment settings to set cell size to finer resolution raster and do substraction. Result might be ugly if correction value vary much from cell to cell. – FelixIP Jun 17 '19 at 06:57

2 Answers2

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One approach is to resample your second raster (elevation correction) to fit the size of your DEM. Of course, as you are going from a bigger pixel size to a smaller one, you will have repeated values for each original pixel (see picture below).

enter image description here Image taken from Chris Garrard's Geoprocessing with Python

On ArcGIS, you can use the Resample tool and use your DEM as the Cell Size parameter, to match the pixel size and then do the substraction using the Raster Calculator.

On QGIS, you can try the first option proposed here, which relies only in the use of the raster calculator.

Marcelo Villa
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A way to approach this issue is the following:

  1. Use QGIS'Create Grid tool (Vector ->Research tools) to create a vector file representing the grid of your coarse resolution DEM (in Grid type select rectangular and in Grid extend select the DEM).
  2. Use the zonal statistics tool to calculate the mean elevation for each polygon of your grid (use as raster input your high-resolution data).
  3. Convert your grid from vector to raster using QGIS' Rasterize tool (Raster -> Conversion -> Rasterise); make sure to use the width and height of your coarse-resolution DEM (in pixels).
pan
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