These instructions assume you have a digital elevation model (DEM) loaded into your ArcView View. 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), there is an extension called Grid Projector available from ArcScripts (from ESRI - go to Avenue or ArcView links). This extension is already loaded on the Sutton Lab GIS computers.
About the Derive Aspect request in ArcView
When the Spatial Analyst extension is loaded into ArcView, two new menu items appear in the View window: Analysis and Surface. One of the functions on the Surface menu is Derive Aspect. This menu function will derive a aspect grid showing direction of slope in terms of 360 degrees of the compass. For example a cell with a value of 90 would be facing straight east. It will only be accurate if both your x,y horizontal units and your elevation units (z units) are of the same type (e.g., all feet or all meters). If they are in different units, which is often the case, the Derive Aspect function will give you inaccurate results. In that case, look at the following instructions.
The following example is for a Austin area DEM imported from TNRIS. These DEMs are usually in the UTM NAD 27, Zone 14, meters, coordinate system.
First re-project the grid into Texas State Plane, NAD 83, Central Zone, feet, using the Grid Projector or Grid Analyst extensions (both of these are available for free download from the ESRI User Scripts web site). . This will convert the horizontal units from meters to feet, but your elevation data (z units) still needs to be checked. Some of the Austin DEMs have elevation data in meters and some have elevation data in feet. You have to look at the elevation values in the DEM grid to judge which is being used. Values in the 400-800 range are probably feet, much lower values are likely to be meters (the metadata is ambiguous unfortunately).
Check the elevation data - if it is in feet, you can go ahead and use the Surface - Derive Aspect with no further adjustments. Make sure you are doing this with the original DEM grid highlighted as the active theme.
If the elevation (z units) are in meters, you need to use the Map Calculator under the Analysis menu to create a new DEM with feet values. To do this:
Make sure the elevation grid is the active theme.
Choose Analysis - Map Calculator
Double-click on the name of the grid in the left-hand column (not the "count" name) so that it shows up in the text box below
Then click on the multiplication symbol (the asterisk * symbol) so that it also shows up below
Then type in 3.281 (the conversion factor for going from meters to feet).
Your final map calculator dialog should look something like this figure
Finally press the Evaluate button. This will calculate and then load a new grid where every grid cell's value is expressed as 3.281 multiplied by the original value, giving you a grid showing z units in feet.
From this point with x,y, and z units all in the same units, you can use all the surface functions like derive slope, aspect, etc. without any more modifications.
Please note that all these grids are temporary! To save them permanently, please follow the instructions for Managing and Saving Grids.
See Peter Wallin's Conversion Table for conversion formulas for every unit of measurement.
