Emma,
I think one way you can do it is when you convert the polygon (NWI shp) to a raster through the tool you can set the processing extent the already created DEM raster through the "environments.." button to match your other rasters. There should be a box you can click where it will snap to the raster.
I think one way you can do it is when you convert the polygon (NWI shp) to a raster through the tool you can set the processing extent the already created DEM raster through the "environments.." button to match your other rasters. There should be a box you can click where it will snap to the raster.