Slicing

Functionality

Slicing classifies the values of a raster map. Ranges of values of the input map are grouped together into one output class. A domain Group should be created beforehand; it lists the upper boundaries of the groups and the group names.

With Slicing, a map with for example slope values ranging from 0 to 200% can be grouped into relief classes: Flat (0-2%), Undulating (2-8%), Rolling (8-16%), Hilly (16-30%), Mountainous (>30%) (FAO). Also, a map containing Green Vegetation Index values (combination of satellite bands), can be sliced into user-defined intervals. This can be considered as a first classification.

Example:

A map with slope values ranging from 0 to 200% is grouped into relief classes: Flat (0-2%), Undulating (2-8%), Rolling (8-16%), Hilly (16-30%), Mountainous (>30%) (FAO).

Input map:

Output map:

6	10	20	40
4	12	20	25
3	7	12	10
1	5	5	5

U	R	H	M
U	R	H	H
U	U	R	R
F	U	U	U

In the output map, read for F Flat, for U Undulating, for R Rolling, for H Hilly and for M Mountainous.

The Slicing is performed according to the following domain Group:

Upper Boundary	Group Name
2	Flat
8	Undulating
16	Rolling
30	Hilly
200	Mountainous

Input requirements:

The input map should be a value map; further, a Group domain should be available.

Domain and georeference of output map:

The output map uses the Group domain that is required for the Slicing operation.

The output map uses the same georeference as the input map.

Tip 1:

The Slicing operation creates a new output map with new class names. Instead, you can also slice/classify a value map by creating and editing a representation for that map. You can then interactively modify the representation and redraw the map until satisfied:

Double-click a value map in the Catalog; in the Display Options dialog box, click the create button next to the Representation list box to create a representation for this map;
Type a name for the new representation; then the Representation Value/Gradual editor is opened;
Click OK in the Display Options dialog box of the map; the map is displayed in a map window;
In the Representation Value/Gradual editor, insert boundaries and adjust colors;
Keep the editor open and switch to the map window;
Press the Redraw button in the map window and the map is directly shown with this representation;
Switch to the Representation editor and continue to change boundaries, and colors; then switch to the mapp window to redraw the map, etc. until satisfied.

Slicing by modifying a representation takes much less time than (re-)calculating values as the Slicing operation would do; another advantage is that the exact input values are retained.

For more information, see How to slice an image.

Tip 2:

To get the best out of the Slicing operation:

When creating a Group domain, also fill the representation of this domain.
Keep the domain window, representation window and output map window open simultaneously; you can modify classes, i.e edit the domain Group, and change its representation;
Open the Properties of the map (Edit menu in the map window, Properties) and click the Make Up-to-Date button; the map is recalculated according to the new boundaries in the domain Group;
Redraw the map in the map window.

Notes:

For ILWIS 1.41 users: a domain Group is a synonym for an ILWIS 1.41 classify table.
Instead of using the Slicing operation to apply a domain Group on a map, you can also use the MapCalc function CLFY(InputMap,DomainGroup) on the command line.
To perform a multi-spectral image classification, see the Classify operation.

Reference:

FAO, 1990, Guidelines for soil profile description. FAO, Rome.