Planning Analyses Using Raster (Grid) Data and Techniques

Working with Digital Elevation Models (DEM) - Merging, Re-projecting, Creating Shaded Relief Maps

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See Importing a DEM for Use in ArcView Spatial Analyst for more information about DEMs, download sources, importing them into ArcView, etc.

Tips for working with digital elevation models

The various surface functions under the surface menu in Spatial Analyst assume that your horizontal coordinates (x,y) are in the same units as your elevation values (z coordinates). Examine the DEM metadata and the DEMs themselves to determine the coordinate system, map units and elevation units. If necessary, use the map calculator to ensure that the x, y, and z elevation values on the all the DEMs are in the same units (e.g., all feet or all meters). If you  had a DEM called Austin-w and you wanted to change the elevation units from meters to feet, for example, you would enter the following equation in the Map Calculator (there are 3.281 feet in a meter - see Peter Wallin's Conversion Table for other conversion numbers):

([Austin-w])*3.281.AsGrid

You can use Grid Analyst 1.0 or 1.1 (from the ESRI's ArcScripts web site, may already be loaded on lab computers) to merge DEM's together.

To show the grid with data in a different coordinate system, use  Grid Analyst or Grid Projector (from the ESRI's ArcScripts web site, may already be loaded on lab computers) to re-project the grid (the other projection utilities that come with ArcView only work on shape files). Remember Austin and Round Rock data is in Texas State Plane, NAD 83, Central Zone. Street data for the rest of Texas is available on the Taniguchi GIS data server in decimal degrees ("geographic" projection).

To create a nice shaded relief map:

1. With your DEM as the active theme, create a hillshade grid (choose Surface - Compute Hillshade)
2. Double-click on the DEM's legend to bring up the legend editor
3. In the legend editor, click on the Advanced button and choose the Hillshaded grid as the brightness theme. Experiment with the cell brightness numbers to test the results.
4. Also experiment with your number of classes in the legend, and with the shades. Make sure your legend type is set to Graduated Color. If you set the first and last colors, you can use the Color Ramp icon () to ramp the colors between the beginning and end colors.  See the sample hillshade map of the Austin region, and then look at the legend editor settings used to create this map.

For more detailed information, see New Color System Enhances Relief Maps, ArcUser Magazine (January-March 2001 issue). This article discusses the new ShadeMax color system that Nat recommended in class. The easiest way to find the ShadeMax download on ESRI's ArcScripts web site is to do a search for shademax.

See Creating a Layout for basic information about ArcView map layouts and printing in Sutton Lab.

Before making slope maps, see Creating a Slope Map from a DEM for important information.

Additional resources for shaded relief mapping:

• NACIS Shaded Relief Home Resources (good site with links to Photoshop tutorials and examples, from the North American Cartographic Information Society - check out their student map contest as well)
• CommEn Space (The Community & Environment Spatial Analysis Center, a non-profit mapping group in the Pacific Northwest  that uses ArcView to do some lovely and informative maps )
• U. S. Geological Survey, Flagstaff Field Center, DEM Shaded Relief Maps

29 March 2001
The School of Architecture at UT Austin
Comments to: Barbara Parmenter