These instructions assume you have a digital elevation model (DEM) loaded into ArcMap. If not, see Importing a USGS DEM for Use in ArcView Spatial Analyst for more information. If you don't have a DEM, but you do have a contour lines shape file and 3D Analyst, see Creating a 3-D TIN elevation model for instructions on how to first create a TIN and then a slope or aspect grid.
If you need to re-project the DEM into a different projection (e.g., into Texas State Plane, NAD 83, Central Zone, feet, to use with Austin or Round Rock data), use the grid projection tools in ArcToolbox.
Understanding x, y, and z units when using Spatial Analyst
The Surface Analysis menu function include Slope, Aspect, and Hillshade. To create accurate derivations, the Surface menu functions require that the x and y horizontal units and your elevation units (z units) to be all the same type (e.g., all feet or all meters). If they are in different units, which is often the case, the they will give you inaccurate results. The horizontal units are the same as the map units of the coordinate system in which the data is projected. The metadata should also tell you what the z (vertical) units are, but sometimes the metadata does not tell you! Or, as in the case of the TNRIS DEMs it will say that the z units are either in meters or feet and you have to figure it out!
If your horizontal x-y units and z units are in different units (e.g., x,y are in meters and z units are in feet) you can use the Z-factor option in the surface analysis tools to do the correction. In the example given (x,y are in meters and z units are in feet), your Z-factor would be 0.3048. This is the number you have to multiply feet by to convert to meters, and that is exactly what the Z-factor does. (See screen print example)
Please note that by default these grids are temporary! To save them permanently, right-click on the new data layer (e.g., calculation) and choose Make Permanent.
See Peter Wallin's Conversion Table for conversion formulas for every unit of measurement - go to View Tables - Unit Conversion Factors. If you look for feet => meters, you will see the conversion factor is .3048
