# Re: [ferret_users] EOF: area weighting?

```Dear Billy and other ferret users interested in EOFs,

weighting for EOF analysis I recommend the following recent paper:

Baldwin, Mark P.; Stephenson, David B.; Jolliffe, Ian T., 2009.	Spatial
Weighting and Iterative Projection Methods for EOFs, Journal of Climate,
22: 234-243, DOI: 10.1175/2008JCLI2147.1

cheers,
Andreas

========================================
Andreas Baumgaertner

Max Planck Institute for Chemistry
Department of Atmospheric Chemistry
Earth System Modelling Group
Joh.-Joachim-Becher-Weg 27
55128 Mainz

Phone: (+49 6131) 305-332
========================================

William S. Kessler wrote:
> Hi Andreas -
>
> Good question. The area represented by each gridpoint should indeed be
> taken into account in computing an EOF. However, in the present EOF
> functions, it is not.
>
> You should therefore do the weighting explicitly before computing EOFs.
>
> But is your expression for the weighting (cos(y/(180/pi))^(0.5))
> correct?
>
> The distance along latitude y1 from x1 to x2 is (x2-x1)*cos(y1).
> The area of a thin strip dy along y1 is (x2-x1)*cos(y1)dy.
> To find the area between y1 and y2, integrate this in y:
>
> A = (x2-x1)INT_y1^y2(cos(y))dy = (x1-x2)(sin(y2)-sin(y1))
>
> Regards .... Billy K
>
>
>
> On Dec 7, 2009, at 3:45 AM, Andreas Baumgaertner wrote:
>
>> Hi ferret users,
>> I am using the functions EOF_SPACE, EOF_TFUNC and EOF_STAT to
>> calculate
>> EOF of geopotential height. This calculation requires equal-area grid
>> box sizes, i.e. a latitude weighting with cos(y/(180/pi))^(0.5). Is
>> this
>> already accounted for in these functions, or do I need to do that
>> before
>> using them?
>> Thanks,
>> Andreas Baumgaertner
>>
>

--
========================================
Andreas Baumgaertner

Max Planck Institute for Chemistry
Department of Atmospheric Chemistry
Earth System Modelling Group
Joh.-Joachim-Becher-Weg 27
55128 Mainz

Phone: (+49 6131) 305-332
========================================

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