r.kappa - GRASS GIS manual
Author: GRASS Development Team
Description: r.kappa: Calculates error matrix and kappa parameter for accuracy assessment of classification result.
Keywords: raster, statistics, classification

## NAME

r.kappa - Calculates error matrix and kappa parameter for accuracy assessment of classification result.

## KEYWORDS

raster, statistics, classification

## SYNOPSIS

r.kappa
r.kappa --help
r.kappa [-whm] classification=name reference=name [output=name] [title=string] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

-w
  Wide report
  132 columns (default: 80)
-h
  No header in the report
-m
  Print Matrix only
--overwrite
  Allow output files to overwrite existing files
--help
  Print usage summary
--verbose
  Verbose module output
--quiet
  Quiet module output
--ui
  Force launching GUI dialog

### Parameters:

classification=name [required]
  Name of raster map containing classification result
reference=name [required]
  Name of raster map containing reference classes
output=name
  Name for output file containing error matrix and kappa
  If not given write to standard output
title=string
  Title for error matrix and kappa
  Default: ACCURACY ASSESSMENT

## DESCRIPTION

r.kappa tabulates the error matrix of classification result by crossing classified map layer with respect to reference map layer. Both overall kappa (accompanied by its variance) and conditional kappa values are calculated. This analysis program respects the current geographic region and mask settings.

r.kappa calculates the error matrix of the two map layers and prepares the table from which the report is to be created. kappa values for overall and each classes are computed along with their variances. Also percent of comission and omission error, total correct classified result by pixel counts, total area in pixel counts and percentage of overall correctly classified pixels are tabulated.

The report will be write to an output file which is in plain text format and named by user at prompt of running the program.

The body of the report is arranged in panels. The classified result map layer categories is arranged along the vertical axis of the table, while the reference map layer categories along the horizontal axis. Each panel has a maximum of 5 categories (9 if wide format) across the top. In addition, the last column of the last panel reflects a cross total of each column for each row. All of the categories of the map layer arranged along the vertical axis, i.e., the reference map layer, are included in each panel. There is a total at the bottom of each column representing the sum of all the rows in that column.

## NOTES

It is recommended to reclassify categories of classified result map layer into a more manageable number before running r.kappa on the classified raster map layer. Because r.kappa calculates and then reports information for each and every category.

NA's in output file mean non-applicable in case MASK exists.

The Estimated kappa value in r.kappa is the value only for one class, i.e. the observed agreement between the classifications for those observations that have been classified by classifier 1 into the class i. In other words, here the choice of reference is important.

It is calculated as:

kpp[i] = (pii[i] - pi[i] * pj[i]) / (pi[i] - pi[i] * pj[i]);

where=

- pii[i] is the probability of agreement (i.e. number of pixels for which there is agreement divided by total number of assessed pixels)
- Pi[i] is the probability of classification i having classified the point as i
- Pj[i] is the probability of classification j having classified the point as i.

## EXAMPLE

Example for North Carolina sample dataset:

    g.region raster=landclass96 -p
    r.kappa -w classification=landuse96_28m reference=landclass96

    # export Kappa matrix as CSV file "kappa.csv"
    r.kappa classification=landuse96_28m reference=landclass96 output=kappa.csv -m -h

Verification of classified LANDSAT scene against training areas:

    r.kappa -w classification=lsat7_2002_classes reference=training

## SEE ALSO

g.region, r.category, r.mask, r.reclass, r.report, r.stats

## AUTHOR

Tao Wen, University of Illinois at Urbana-Champaign, Illinois

## SOURCE CODE

Available at: r.kappa source code (history)

Accessed: Sunday Jun 21 14:58:41 2026

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