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 Slope 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 Slope.  This menu function will derive a slope grid showing degree of slope. 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 Slope function will give you inaccurate results.

You can use the Map Calculator under the Analysis menu to deal with this, and also to derive a percent slope map. But first, you need to understand how the slope request works.  The slope request in Avenue (ArcView's scripting language) is represented generically as follows:

.slope(zFactor,percentRise)

The .slope is the actual Avenue request, and the two objects in parentheses are arguments for this request.

The zFactor defines the number of x,y units in one z unit. By default it is set to "Nil" which means there is no z factor - one unit of x,y (horizontal coordinates) is equal to one unit of z (elevation). You could also specify this as the number 1.

If your x,y units are in meters, and your elevation units are in feet (as is sometimes the case with the TNRIS DEMs for Texas), you need to set the zFactor as 0.3048.  There are 0.3048 meters in a foot. If you have already re-projected your DEM into a different coordinate system where the unit is feet (e.g., Texas State Plane), and your elevation unit is still meters, then you would use 3.281 as the zFactor because there are 3.281 feet in a meter.

In place of percentRise, you need to specify either TRUE or FALSE. If you specify FALSE (as the menu function for Derive Slope in ArcView does) then you will get a slope grid showing degree of slope. If you specify TRUE, you will get a slope grid showing percent of slope.

Using the Slope Request in the Map Calculator

You can use Avenue requests like .slope in the Map Calculator under the Analysis menu. Just pay close attention to your brackets and parentheses. The grid name must be in brackets, and the two arguments must be within a set of parentheses as show below:

• If a DEM grid called austin-elev has x,y units and z units of the same type, and you want a percent slope grid, you would choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(Nil,TRUE)
  

• If a DEM grid called austin-elev has x,y units in feet and z units in meters, and you want a percent slope grid, you would choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(3.281,TRUE)
  

• If a DEM grid called austin-elev has x,y units in feet and z units in meters, and you want a degree slope grid, you would choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(3.281,FALSE)
  
  

• If a DEM grid called austin-elev has x,y units in meters and z units in feet, and you want a percent slope grid, you would choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(0.3048,TRUE)
  

• If a grid called austin-elev has x,y units in meters and z units in feet, and we want a degree slope grid, you would choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(0.3048,FALSE)
  

• Finally, if a grid called austin-elev has x,y units and z units of the same type, and you want a degree slope grid, you could choose Analysis - Map Calculator and type:
  
  [austin-elev].slope(Nil,FALSE)
  
  In this last case, you don't even have to use the Map Calculator, because by default, ArcView's Derive Slope menu function is .slope(Nil,FALSE), so that when you make a DEM grid active, and choose Surface - Derive Slope, you are assuming x,y, and z units all of the same type and getting a slope grid in degrees.

 See Peter Wallin's Conversion Table for conversion formulas for every unit of measurement.