Shaded relief rasters (also called hillshade) are frequently used for visualization and cartographic purposes and are calculated from a DEM and predefined values for the azimuth (horizontal deviation from north, clockwise) and the solar elevation angle (vertical inclination between the sun and the horizon). The Hillshade (Spatial Analyst or 3D Analyst) tool calculates the shaded relief of the terrain in ArcGIS. This tool is useful for determining which areas of the terrain are shaded and which are not, during certain hours of the day, month, or year.
To calculate the hillshade of the terrain (Fig. 45):
- Type “hillshade” into the Search window (or browse to Spatial Analyst Tools > Surface > Hillshade in the ArcToolbox).
- Select the Hillshade (Spatial Analyst or 3D Analyst) tool.
- Choose the DEM as the input raster.
- Save the output raster as aspect.
- Click OK.
The results will be shown in a greyscale with values ranging from 0 to 254 (Fig. 46), where 0 is black, representing completely shaded areas, 254 is white, representing illuminated areas, and all values in between representing an increasingly lighter shade of grey, thus different levels of illumination from the sun.
Hillshade rasters are good for visualizing terrain under different layers. To display the terrain properties under the DEM:
- Drag the hillshade layer to the bottom of the layer list in the TOC.
- Turn off all of layers except for the hillshade and the DEM.
- Open the properties of the DEM, right-click DEM layer.
- Select Display.
- Set the transparency to 50%.
- Go to the Symbology tab.
- Select a different colour ramp.
- Click OK.
The resulting image (Fig. 47) enables a more intuitive view of the
terrain with high elevations in red, low elevations in green, and the ability
to see shadows from the hillshade.
Fig. 45. The Hillshade tool dialog box.
Fig. 46. The result of the hillshade calculation from the DEM.
Fig. 47. Example of a transparent DEM draped over the hillshade raster to show terrain definition.