How to calculate an aspect map

1.

Create a segment map with a value domain. 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 (DEM).

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 an aspect map in radians from these maps DX and DY, type on the command line of the Main window:

ASPECTR = ATAN2(DX,DY) + PI

To convert the radians into degrees, type:

ASPECTD = RADDEG((ASPECTR))

If you wish, you can combine both statements above into one:

ASPECTD = RADDEG(ATAN2(DX,DY) + PI)

Functions ATAN2 and RADDEG are internal MapCalc/TabCalc functions. ASPECTR and ASPECTD are the output map names of the aspect maps in radians and degrees respectively.

6.

To classify the aspect map, first create a domain group (e.g. called ASPGR) in which you can use for instance the following boundary values and classes:

22.5

North

67.5

North-East

112.5

East

157.5

South-East

202.5

South

247.5

South-West

292.5

West

337.5

North-West

361

North2

7.

Then, use the Slicing operation. From the Operations menu in the Main window, choose Image Processing, Slicing: use as input map ASPECTD, as the group domain APSPGR and call the output map for instance ASPECTCL.

On the command line, you can use:

ASPECTCL = MapSlicing(ASPECTD,ASPGR)

8.

Adapt the representation class ASPGR which accompanies group domain ASPGR according to your wishes; use the Representation Class editor.