[R] correlation value and map

Jenny Barnes jmb at mssl.ucl.ac.uk
Wed Jan 10 14:43:20 CET 2007

Dear R-help community,

I have 2 different arrays of precipitation data each of the same dimensions of 
[longitude, latitude, time] dim=[30,32,43], called array1 and array2. I need to 
correlate them. This is the code I used to get one overall correlation value for 
the whole of the area of interest:

> result <- cor(array1,array2,use="complete.obs")
> result

This give me a single value but I'm not convinced it is actually a correlation 
value for the total area for the total time period of 43 years....can anybody 
tell me if I am indeed wrong in my coding and/or indeed my low knowledge of the 
statistics of correlation.

Also, I wanted to produce a correlation map over the 43 years. Could you also 
advise me if this is correct, I am more confident that this is than the above 

> result <- array(NA, c(30,32))
> for(i in 1:30){
> for(j in 1:32){
>	array1.ts <- array1[i,j,]
>	array2.ts <- array2[i,j,]
>	result[i,j] <- cor(array1.ts,array2.ts,use= "complete.obs")
> }
> }

I appreciate your time very much. If I don't iron out this problem now the 
ground-work for my entire PhD will not be stable at all,

Many thanks for reading my problem, happy 2007 :-)

Jenny Barnes

Jennifer Barnes
PhD student - long range drought prediction
Climate Extremes
Department of Space and Climate Physics
University College London
Holmbury St Mary, Dorking
01483 204149
Web: http://climate.mssl.ucl.ac.uk

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