calculates the first derivative in x-direction (df/dx) per pixel
We already have a DEM, we can directly start the filtering procedure. Select from the main ILWIS menu the option “Operation” > “Image Processing” > “Filtering”, as Raster Map “eth_demfin”, as filter name select “DFDX” from the drop down menu, as output map specify “DX”
see also : DFDX Filter