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What are the units of measure for Hillshade

January 15 2011 | 0 comments
Categories: Map Elements

I am a member of T3G2009 and asked my colleagues the following questions but was referred to your website for expert advice! Can you help me?
1. When someone creates a map using both a DEM and its hillshade (shaded relief map), should the hillshade be included in the legend even though the hillshade isn't directly seen in the map? If it was used in the map, I think it should be included so the viewer knows exactly what was used.
2. Hillshade values range between 0-254. The creation of a hillshade involves illumiation, slope, and other data. If I include the hillshade in my map legend what do I use to label the hillshade units of measurement? I will include my map to better explain...

Thank you so much for your help.

Mapping Center Answer:

What a great question! Although I will answer your question, ultimately you will see that we actually advise that you make a custom legend.

In answer to your question, the values in the hillshade range from 0-255 which relate to a range of "illumination" values from zero (no illumination, or in other words, complete shadow) to 255 (full illumination, or in other words, full sunlight). The values in your DEM relate of course to elevation. This is the more important thing for readers to understand. Since the elevation colors are modified by whether the location is illuminated or not, that is why we combine the hillshade and the elevation in one legend and label it with the elevation values only. But this has to be done by creating a custom legend. The legends that are created using the defaults are not usually the best choice for showing a color ramp that depicts elevation over a hillshade that depicts illumination. For these types of legends, we have written up instructions for their creation in a couple of blog entries. These are custom legends that you create yourself. Here are the links to the blog entries:

By the way, I noted that you are using one of the two Elevation color ramps that come with the Esri style. I would like to suggest that you download ColorRamps 2.0, which you can find here on Mapping Center on the ArcGIS Resources - Styles page. In the Hypsometry style, I think you will find something that will work better for your area. Since your mapping a coastal locale, my guess is that the range in elevation is not as great as it appears using the color ramp you currently have. You might be happier with something else. To restrict the range of colors used in the color ramp, use the Stretch Type Minimum-Maximum and check the box to Edit high/low values, then set the high value to a lower number so that you are not getting the colors for the extreme highs in the ramp (or visa versa for the low values).

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