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Re: [ferret_users] Z=@din and z=@ddf

```Hi YangXing -

```
This topic has been discussed several times in the mailing list. Search the mail archives for "rsum", especially see the message from Steve "How to do @rsum and @iin in reverse?". Also see the further reply by Ryo Furue. These were on 14 Jul 2006. Also a similar discussion on 18 Dec 2007.
```
Billy K

On 8May 2008, at 11:12 AM, yangxing zheng wrote:

```
```Hi ferreters,

Today I do have a short question to ask. Probably
it is easy to others. I have an ocean data which
includes vertical velocity W as a function a depth.
The vertical axis is defined as a depth, which means
downward is positive direction. My question is:

What is the physical meaning if I integrate w
along vertical coordinate, w[z=0:300@din], it means
integrate from surface (0) to 300 meter depth, right?
The value of w[z=0:300@din] is the minus of that with
reverse integral direction?

Similar question is the difference between
w[z=0:300@ddf] and that with reverse direction.

For example:

depth (m)       W value (cm/s)

0                    0.000

10                  0.007

20                  0.008

30                   0.011

then w[z=0:30@ddf] in ferret is calculated in this
way?

at surface: (w2 - w1)/(z2-z1)= (0.007 -
0.000)/(10 - 0)

at 10m depth: = (0.008 - 0.007)/(20 -10)

They take z coordinate value is always positive
in the ocean interior if it is defined with /depth?

Thanks

Y.Z

```
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