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How To: Create a hillshade or slope using data in Geographic coordinates

Summary

When performing a hillshade or slope function in ArcGIS, the Z units are presumed to be the same as the horizontal (x,y) units. When working with data in geographic coordinates, this is a problem because the linear distance of a degree of latitude varies.

In some cases, a value of 0.000003 was suggested as the Z factor to account for the actual Z unit in meters. While that value can still be used, this article presents a more accurate method of determining the Z factor when the Z units are in meters.

Procedure

The following procedure assumes the Z units of the raster being used to generate the hillshade or slope are meters. If the Z units of the data being used are feet, first use the raster calculator to multiply the raster by the conversion factor 0.3048, so that the z units are meters.

  1. Determine what the middle latitude of the area of interest is.
  2. Convert that degree value to radians.
    1 degree = 0.0174532925 radians
  3. Use the value in radians in the following equation:
    Z factor = 1.0 / (113200 * cos(<input latitude in radians>))
  4. Use this calculated Z factor in the hillshade or slope tool.