Re: [ferret_users] z axis spacing

On Tue, Aug 10, 2010 at 6:34 PM, Marco Steinacher <steinacher@xxxxxxxxxxxxxxxx> wrote:

Dear Sudheer,

I think the values you get for ZBOX make sense. Think of your
measurements as they were taken at the center of grid cells and ZBOX(Z)
is the height of the box with the center at depth Z. If no explicit edge
definition is given Ferret takes the midpoints between coordinate values
as edges. This leads to the results you have got. Here's a picture:

Z Edges ZBOX
...
|
296.5
298 -- | --> 3.0
299.5
301 -- | --> 4.0
303.5
306 -- | --> 4.5
308.0
310 -- | --> 7.0
315.0
320 -- | --> 10.0
325.0
330 -- | --> 10.0
335.0
340 -- | --> 10.0
345.0
350 -- | --> 15.0
360.0
370 -- | --> 20.0
...

Z = center of grid cells
Edges = Midpoints between Z (assumed by Ferret)
ZBOX = height of grid cell

In many cases this makes sense. For example if you want to calculate the
vertical integral you assume that your measurement is the average of
some space above and below the point where it was taken. If you have no
other information it makes sense to take the midpoints between the
measurements.

If you want other values for ZBOX for some reason you have to specify
the edges of your grid boxes explicitly by using the /bounds qualifier
with the define axis command (see Ferret documentation).

Marco

Sudheer Joseph wrote:
> Dear Users,
>
> I am trying to get data sampled at every 3 meters in upper 3
> hundred meters and then switching to 30 meters in few steps of 5 10 20
> for that I used the pasted below depth intervals.
> and below commands to define the grid.
>
> file/var=tt tt
> def axis/z/from_data/depth/units=meters zax tt[d=1]
>
> define grid/z=zax ngrd
> file/var=var_1/grid ngrd
>
> But If I list the zbox and corresponding z values I get some thing
> which I did not expect. Can any one comment on this? is it a bug or am
> I using ferret in a way I am not supposed to do?
> I get un expected values for zbox with 15 against z values of 10
> meter gap!! How this can happen?
> 4.5 may be expected as depth adjustment between 301 and 306 but how
> can it be expected to see 15 meter depth between 340 and 350?
>
> yes? list zbox[gz=zax],z[gz=zax]
> DEPTH (m): 278.5 to 625
> Column 1: ZBOX is ZBOX (axis ZAX)
> Column 2: Z is Z (axis ZAX)
> ZBOX Z
> 280 / 1: 3.00 280.0
> 283 / 2: 3.00 283.0
> 286 / 3: 3.00 286.0
> 289 / 4: 3.00 289.0
> 292 / 5: 3.00 292.0
> 295 / 6: 3.00 295.0
> 298 / 7: 3.00 298.0
> 301 / 8: 4.00 301.0
> 306 / 9: 4.50 306.0
> 310 / 10: 7.00 310.0
> 320 / 11: 10.00 320.0
> 330 / 12: 10.00 330.0
> 340 / 13: 10.00 340.0
> 350 / 14: 15.00 350.0
> 370 / 15: 20.00 370.0
> 390 / 16: 15.00 390.0
> 400 / 17: 20.00 400.0
> 430 / 18: 30.00 430.0
> 460 / 19: 30.00 460.0
> 490 / 20: 30.00 490.0
> 520 / 21: 30.00 520.0
> 550 / 22: 30.00 550.0
> 580 / 23: 30.00 580.0
> 610 / 24: 30.00 610.0
>
> --
> depths in file tt
> $ cat tt
> 280
> 283
> 286
> 289
> 292
> 295
> 298
> 301
> 306
> 310
> 320
> 330
> 340
> 350
> 370
> 390
> 400
> 430
> 460
> 490
> 520
> 550
> 580
> 610
>
>
>
>
> with best regards
>
> Sudheer
>
> **********************************************************************************
> Sudheer Joseph
>
> Scientist
>
> Indian National Centre for Ocean Information Services (INCOIS)
> Ocean Valley, Post Box No# 21,
> IDA Jeedimetla P.O.
> Hyderabad, Ranga Reddy District - 500 055
> Andhra Pradesh, India.
> TEl:+91-40-23044600(R),Tel:+91-9440832534(Mobile)
> Tel:+91-40-23886047(O),Fax:+91-40-23892910(O)

> E-mail:sjo@xxxxxxxxxxxxxx <mailto:E-mail%3Asjo@xxxxxxxxxxxxxx>;
> sudheer.joseph@xxxxxxxxx <mailto:sudheer.joseph@xxxxxxxxx>;
> sjo@xxxxxxxxxxxxx <mailto:sjo@xxxxxxxxxxxxx>.

> Web- http://oppamthadathil.tripod.com
> --------------* ---------------
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> he stands at times of challenge and
> controversy."
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--
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Marco Steinacher

Climate and Environmental Physics
Physics Institute, University of Bern
Sidlerstrasse 5, CH-3012 Bern

Phone ++41 (0)31 631 34 02
Fax ++41 (0)31 631 87 42
steinacher@xxxxxxxxxxxxxxxx
http://www.climate.unibe.ch/~steinach
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