reclassification
map algebra
boolean logic in GIS (+ interactive example)
slicing

Reclassification

Reclassification is a generalisation technique used to re-assign values in an input raster layer to create a new raster layer. Reclassification changes the value of the input cells on a cell-by-cell basis within the area under analysis.

This technique is commonly used to convert interval and ratio scale data into an ordinal ranking for landuse suitability modelling using map algebra. It has the additional benefit of reducing file sizes of raster layers.

Map Algebra

Map algebra creates new features and attribute relations by overlaying the features from two input map layers. Features from each input layer are combined to create new output features. Attributes of each input feature are combined from the two input layers to describe each new output feature, thus creating new attribute relationships.

Often, the manipulation of multiple data layers is required to achieve the objective of the overlay operation. This is done in a stepwise fashion - two input layers are combined to form an intermediate layer, this intermediate layer is then combined with a third layer to form another intermediate layer, and so on until the desired resultant map layer is achieved.

Map algebra uses boolean logic.

Boolean Logic in GIS (including Simple Interactive Example)

Boolean Algebra is useful for performing operations on the attributes (which may be positional or descriptive) attached to geographic entities in a GIS. Boolean Logic is especially useful in computing (or modelling) new attributes in topological overlay processing for both vector and raster based systems, as they can be applied to all data types, be they Boolean, Ratio, Interval, Ordinal, or Nominal. Boolean algebra uses the logical operators AND, OR, NOT to determine whether a particular condition is true or false.

Each attribute can be thought of as defining a set. Consider two sets (set A and set B):

The AND operator () is the intersection of two sets - for example those entities that belong to both set A and set B
( A B)

The OR operator () is the union of two sets -
for example those entities that belong to either set A or to set B
( A B)

The NOT operator () is the difference operator identifying those entities that belong to A but not B
( A B)

For a GIS related example of the use of these boolean logical operations on raster datasets, follow this link: Boolean Logic - Simple Interactive Example

slicing

Slicing is a global function similar to reclassification, however it uses statistical measures to subdivide the region from interval/ratio scales to ordinal rankings, rather than user-defined intervals. There are two different approaches to slicing: equal interval and equal area.

Equal area means that the input values will be divided into n zones with each zone having the same number of cells, i.e. so that each zone represents the same area.

The equal interval slice determines the range of the input values and then divides the range into n zones, therefore each zone could have a different area..

Slicing is useful to simplify displays or analysis of continuous data such as elevation and rainfall. It is also used to reduce the number of categories in an interval level map.

Slice changes a range of values of the input cells by specified ranges, zones of equal area, or zones with equal intervals within the analysis window