How to calculate a slope map

To calculate slope maps in percentages and in degrees:

1.	Create a segment map with a value domain (it is advised to create a value domain Height). Digitize the contour lines and give the contour lines the height value they represent.
2.	To create an interpolated height map from segment contour lines: open the Operations menu in the Main window, choose Interpolation, then choose Contour Interpolation. The output map is a Digital Elevation Model.
3.	To calculate height differences in X-direction: start the Filter operation, select the Digital Elevation Model as the input map and select linear filter dfdx. Call the output map for example DX.
4.	To calculate height differences in Y-direction: start the Filter operation again, select the Digital Elevation Model as input map and select linear filter dfdy. Call the output map for example DY.
5.	To calculate a slope map in percentages from these maps DX and DY, type on the command line of the Main window: SLOPEPCT = 100 * HYP(DX,DY)/ PIXSIZE(DEM) HYP is an internal Mapcalc/Tabcalc function. Function PIXSIZE returns the pixel size of a raster map; for DEM, fill out the name of your DEM created in step 2. SLOPEPCT is the output map name of the slope map in percentages.
6.	To convert the percentage values into degrees, type: SLOPEDEG = RADDEG(ATAN(SLOPEPCT/100)) Function ATAN and RADDEG are internal MapCalc/TabCalc functions. SLOPEDEG is the output map name of the slope map in degrees.

Mind: the following slope values are the same: 30�= 58%, 45�= 100%, 60�= 173%, 80�= 567%. As you see, slope values in the SLOPEPCT map can be greater than 100%.

Tips:

When you start creating the segment map for the contour lines, it is advised to create a user-defined value domain Height. Afterwards you can then create a user-defined representation Height for this domain. You can change the colors of this representation in the Representation Value/Gradual editor.
You can also create a script to perform steps 2 through 6 described above. For more information, see Map & Table calculation : creating and running scripts.
To classify the values of the slope maps, there are two possibilities:

Create a new representation value for the output slope map and adapt the colors in the Representation Value/Gradual editor. For more information, see How to classify a map.
Create two domain Groups, for example: classes 0-10%, 10-25%, 25-50%, 50-100%, >100% for the slope map in percentages and classes 0-6�, 6-12.5�, 12.5-22.5�, 22.5-45�, >45� for the slope map in degrees. Then, use these domain groups in the Slicing operation.