[GRASS-user] Fwd: [GRASS-web] r.kappa help

Moritz Lennert mlennert at club.worldonline.be
Thu Feb 23 01:42:05 PST 2017


[Resonding to a question on grass-web]

> ---------- Forwarded message ----------
> From: rosa.coluzzi <rosa.coluzzi at imaa.cnr.it>
> Date: Wed, Feb 22, 2017 at 6:11 PM
> Subject: [GRASS-web] r.kappa help
> To: grass-web at lists.osgeo.org
>
>
> Greetings,
>
> please, can you help me to understand what's exactly the "Estimated
> Kappa value" in output of r.kappa routine?

The kappa coefficient measures the agreement between two classifications 
by taking the observed agreement between the two classification and 
subtracting from that the probability that the agreement is only by chance.

Normally, this is calculated across all classes and thus is not 
conditional to which classification you chose as reference.

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 (taken from the code [1]):

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

where=

pii[i] is the probability of agreeement (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

Hope this helps.

Moritz



[1] 
https://trac.osgeo.org/grass/browser/grass/branches/releasebranch_7_2/raster/r.kappa/calc_kappa.c


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