Re: ncdigest V1 #419

Harvey DAVIES (hld@dit.csiro.au)
Thu, 7 Aug 1997 14:57:51 +1000 (EST) 

On Fri, 1 Aug 1997 owner-ncdigest@unidata.ucar.edu wrote:

> ncdigest                  Friday, 1 August 1997      Volume 01 : Number 419

> From: jps@gfdl.gov (John Sheldon)
> Date: Fri, 1 Aug 1997 19:39:29 -0400 (EDT)
> Subject: Re: More Songs about Coordinate Systems and Buildings

> >   8. correlations, using a dimension more than once:
> >       precip(time, npoints)
> >       precip.correlation( npoints, npoints)
> >           lat(npoints)
> >           lon(npoints)
> 
>   Your solution:
> >       - will need some notation not yet formally proposed, eg:
> >           :coordinates = "lat(npoints,) lon(npoints,) lat(,npoints)
>   My solution:
>       Not applicable..."correlation" does not possess "coordinates" the
>       way we think of them.  [rest deleted]

I just want to emphasise my agreement by pointing out that one could
equally well be considering correlations between different variables at
the same station.  This is a little nasty in that you need a data matrix
with different units of measure for each column!  E.g. say we have matrix
         data(time,var)
where 
var=0 is annual precipitation, 
var=1 is annual mean temperature,
var=2 is annual mean relative-humidity, 
etc.

Then  corr(var,var) is correlation matrix.  
So corr(0,2) is correlation between variables:
1. annual precipitation 
and
2. annual mean relative-humidity.

You could even combine both stations and variables.  E.g. start with data array
         data(time,var,station)
and calculate 4D correlation array corr(var,station,var,station).  
So corr(0,5,2,9) is correlation between variables:
1. annual precipitation at station 5
and
2. annual mean relative-humidity at station 9.