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What if I am having problems creating shaded relief images?

Creating shaded relief directly from seamless DEMs can be problematic, because the horizontal units are in decimal degrees but vertical units are in meters. When horizontal and vertical units aren't the same, most hillshading functions require extra care in specifying an exaggeration (or “Z”) factor. Depending on the software used to generate a raster hillshade, the data may need to be projected before building the hillshade.

There are two common methods for creating a hillshade from the seamless DEMs. The first method requires projecting the data to a rectangular coordinate system and converting the elevation units to match the horizontal units. The second method requires the application of a “Z factor” when the hillshade is created.

Method 1:  First, project the DEMs to the desired coordinate system. If the horizontal units (x,y) of the desired coordinate system are meters (for example, UTM), no further processing is needed before a hillshade can be created. If the horizontal units of the desired coordinate system are not meters (for example, State Plane (US Feet)), the elevation values (z) must be converted to match the horizontal units. This requires conversion of the actual pixel values of the DEM after the projection is completed. In other words, the projection alone will not provide a DEM suitable for creating a hillshade if the x,y units do not match the z units.

Method 2:  Using the seamless DEMs directly to create a hillshade can be done by applying a “Z factor.” The Z factor can usually be applied as a parameter in the software being used to create the hillshade. The purpose of applying the Z factor is to properly scale the horizontal units (decimal degrees) to match the elevation units (meters). Although the factor for converting decimal degrees to meters varies for different latitudes, a Z factor of approximately 0.00003 should produce reasonable results for most applications.

 Figure 1:  Shaded relief of 1/9 arc-second DEM data created with no Z factor applied Figure 2:  Shaded relief of 1/9 arc-second DEM data created with Z factor of 0.00003

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