Values for hillshade or slope are incorrect when using digital elevation model (DEM) in a geographic projection

Description

When calculating a slope, the output values for the resulting raster are incorrect. The values are usually much greater than expected.

Cause

When ArcGIS calculates a slope or a hillshade, the x and y units (horizontal) are presumed to be the same as the z units (vertical). This causes problems because the linear distance of a degree of latitude varies.

Solution or Workaround

• For data that must be maintained in a geographic coordinate system, follow the steps in this article, How To: Create a hillshade or slope using data in Geographic coordinates.
• For data that does not need to be maintained in a geographic coordinate system, project the data to a coordinate system that does not use decimal degrees or degrees minutes seconds for the x,y units of measure. Go to: ArcToolbox > Data Management Tools > Projections and Transformations > Raster > Project Raster.