Aggregate map

Functionality

The Aggregate Map operation aggregates blocks of input pixels by applying an aggregation function: Average, Count, Maximum, Median, Minimum, Predominant, Standard Deviation or Sum. The Aggregate Map operation either creates a new georeference in which each block of input pixels corresponds to one output pixel (group) or the output raster map uses the same georeference as the input map (no group).

The Aggregate Map operation is designed for applications that require a multi-resolution approach, for instance to find densities of certain features in raster maps. You can also use this operation to generalize raster data.

First, the user has to specify a group size which determines the size of the blocks of input pixels, e.g. 3 x 3, 4 x 4, 5 x 5 pixels, etc. on which an aggregation has to be performed.

Then, you have to select an aggregation function which calculates an output value for each block of (n x n) input pixels:

Average

calculates the average for each block of input pixel values;

Count

counts the number of pixels in each block of input pixels that are not undefined;

Maximum

finds the maximum for each block of input pixel values;

Median

finds the median for each block of input pixels;

Minimum

finds the minimum for each block of input pixel values;

Predominant

finds the most occuring (predominant) class name, ID or value for each block of input pixels;

Std Deviation

calculates the standard deviation for each block of input pixel values;

Sum

calculates the sum for each block of input pixel values.

All aggregation functions work on value maps. The aggregation functions Count, Median and Predominant also work on maps with another domain type. For the Median function, the order of a Class or ID domain is used.

Lastly, you have to choose between Group and No Group:

Figures 1, 2 and 3 below illustrate the difference between Group and No Group.

Example using Group and No Group:

In this example, the input map has 6 x 6 pixels (Figure 1). Each block of 3 x 3 input pixels is aggregated with the Average function. With the Group option, this results in a map of 2 x 2 pixels (Figure 2); with the No Group option, this results in a map of 6 x 6 pixels (Figure 3).

 

Input map:

Fig. 1: Input map for an aggregation.

Output map Group:

Ouput map No Group:

Fig. 2: Output of average aggregation when using the Group option.

Fig. 3: Output of average aggregation when using the No Group option.

Input map requirements:

All aggregation functions can be used on value maps.

The aggregation functions Count, Median and Predominant can also be applied on raster maps with another domain type.

The operation can also be applied on all raster maps in a map list.

Domain and georeference of output map:

The default domain depends on the selected aggregation function.

When the option Group is selected, a new georeference is created for the output map. When the option No Group is selected, the output raster map uses the same georeference as the input raster map.

When the operation is performed on an input map list, you will obtain an output map list. When the output map list is opened, the output raster maps will be (defined and) calculated.

Tips:

See also: