# Re: [ferret_users] EOF: area weighting?

```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:

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```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

```
```

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