R/Kappa.R

In greenbrown: Land Surface Phenology and Trend Analysis

Kappa <- structure(function(
	##title<< 
	## Calculate the Kappa coefficient of two classifications
	
	##description<<
	## This function takes a contingency table as calculated with \code{\link{table}} or \code{\link[raster]{crosstab}} and computes the Kappa coefficient.
	
	tab
	### contingency table as calculated with \code{\link{table}} or \code{\link{crosstab}}
	
	##references<< Congalton, R.G. (1991): A review of assessing the accuracy of classifications of remotely sensed data. - Remote Sensing of Environment 1991, 37, 35-46.

	##seealso<<
	## \code{\link{CompareClassification}}, \code{\link{AccuracyAssessment}}, \code{\link{TrendClassification}}	
) {
	n <- sum(tab)
	rsum <- rowSums(tab)
	csum <- colSums(tab)
	p0 <- sum(diag(tab)) / n
	pc <- 1/(n^2) * sum(rsum * csum)
	k <- (p0 - pc) / (1 - pc)
	return(k)
	### Kappa coeffcient
}, ex=function() {

# two classifications: 
a <- c(1, 1, 1, 2, 2, 2, 3, 4, 5, 5, 5, 1, 1, 1, 2, 2, 2, 3, 4, 5, 5, 3, 3, 2, 2)
b <- c(1, 2, 1, 2, 2, 2, 3, 4, 2, 2, 5, 1, 2, 2, 2, 1, 2, 3, 4, 5, 5, 3, 3, 2, 2)

# calculate first a contingency table
tab <- table(a, b)

# calculate Kappa coeffcient
Kappa(tab)

})