Hi Uli,You may convert a polygon feature into a raster using the Polygon to Raster tool located in the To Raster toolset in the Conversion toolbox. This tool will create a thematic raster based on the numeric or string LU field. The raster will be a grid of pixels (cells) identifying the LU location and value. These pixels have a cell size of a certain defined square area. You may specify your cell size in the Polygon to Raster tool. If you are working with a geodatabase I would recommend creating a geodatabase raster, otherwise a *.tif or *.img format are common to use for this type of data. Below is an exhaustive list of the raster file types that ArcGIS will work with and some documentation on the polygon to Raster tool. Polygon to Raster (Conversion)http://resources.arcgis.com/en/help/main/10.1/index.html#//001200000030000000Supported raster dataset file formatshttp://resources.arcgis.com/en/help/main/10.1/index.html#//009t0000000q000000Cell size of raster data (onine documentation)http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009t00000004000000I'm not sure what your complete workflow is? Is your polygon already aggregated by LU in a grid type areas or do you want to aggregate the LU raster created by the LU polygons into a grid? Best wishes with your project.Carrie