This option allows
you to generate a grid (and optionally areas) of roughness length
values from a land cover layer. When you select this option, the following
dialog appears allowing you to setup the roughness calculation:
Several land cover data sets useful for roughness length calculation, like CORINE, NLCD, and VCF, have built-in roughness conversion tables available.
Custom roughness tables can be defined in a roughness_tables.txt file in the user data file path listed on the Configuration > General > Advanced dialog. A sample roughness_tables.txt file can be found here.