How to calculate a classified aspect map

(Advanced)

An aspect map shows to which side a slope is directed. A classified aspect map does not show the angle in radians or degrees but only a limited number of classes. To calculate an aspect map, refer to How to calculate an aspect map.

When you already calculated an aspect map, you can classify that map by creating a new representation value for the map, or by creating and applying a Group domain on it with the Slicing operation. For more information, refer to How to classify a map.

This topic deals with the direct calculation of a classified aspect map by using a neighbourhood function. Although the commands to use are rather simple, this topic is considered to be for advanced users because of the concepts of neighbourhood operations.

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.
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). Keep in mind the pixel size that you are using.
The DEM is used as the input map in a neighbourhood operation. In the neighbourhood function NBMINP, a pixel is assigned the position number of the lowest value of all neighbours in a moving window of 3 by 3 cells. The pixels in the moving window are coded as follows:

1	2	3
4	5	6
7	8	9

The direction of the slope in the classified aspect map is found according the position of the neighbour with the lowest value:

	Pixel value	Direction
	1	NW
	2	N
	3	NE
	4	W
	5	Flat
	6	E
	7	SW
	8	S
	9	SE

To calculate the classified aspect map, type in the command line of the Main window:

ASPECT = NBMINP(DEM#)

The output map contains values 1, 2, 3, ... 9.

As an example, part of a DEM and the corresponding pixels in the classified aspect map are displayed below.

DEM	ASPECT