You have already downloaded the three required zip files (cf. above under ‘required datasets’). If not, you will find the links to the data also below. 1.1. Unzip cngrid.zip in your working directory. The ArcCatalog-view of the data folder is shown below: 2) cedar_dem is the raw 30 DEM for Cedar Creek obtained from USGS and clipped for the study watershed, 3) cedar_lu is the 2001 land cover grid from USGS. All datasets have a common spatial reference (NAD_1983_UTM_16). 1.2. Unzip terrain_geohms.zip in your working directory. The ArcCatalog-view of the data folder is shown below: 1) cedar_dem is the raw 30 DEM for Cedar Creek obtained from USGS and clipped for the study watershed. Both raster cedar_dem and streams.shp are already assigned a projected coordinate system (NAD_1983_UTM_16). 1.3 Unzip the GeoRASdata.zip file in your working directory. It contains the baxter_tin data required in section 8.2 Open ArcMap. Create a new empty map, and save it as cngrid.mxd (or any other name). Add Spatial Analyst extension and activate it by clicking on Customize –>, Extensions…, and checking the box next to Spatial Analyst. These numbers represent a land use class defined according to the USGS land cover institute (LCI). A description of some of the land classes and their associated numbers can be found here. 6.1. Terrain Preprocessing: Hydro DEM, Flow Direction Grid, Flow Accumulation Grid, Stream Definition Grid, Stream Segmentation Grid, Catchment Grid Delineation. 7.2.3. Extracting Basin Characteristics: River Lenght, River Slope, Basin Slope, Longest Flow Path, Basin Centroid, Basin Centroid Elevation, Centroidal Longest Flow Path The basin characteristics menu in the HEC-GeoHMS Project View provide tools for extracting physical characteristics of streams and sub-basins into attribute tables. After the inundation map is created, you must check the inundation polygon for its quality. You will have to look at the inundation map and the underlying terrain to correct errors in the flood inundation polygon. Sometimes you will realize (at the end!) that your terrain has errors, which you need to fix in the HEC-RAS geometry file. The refinement of flood inundation results to create a hydraulically correct output is not covered in this tutorial – this is an iterative process requiring several iterations between GIS and HECRAS. The ability to judge the quality of terrain and flood inundation polygon comes with the knowledge of the study area and experience. Source.