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Re: [ferret_users] z axis spacing



Hi Sudheer,
An alternate way to define coordinate axes is with the /EDGES qualifier, so that the points you give describe the cell edges rather than the coordinate values. This will probably give you the control you want. You can also define both the edges and coordinates, so that the coordinates do not have to be centered in the grid cells. This is done using the /BOUNDS qualifier. These are all described under DEFINE AXIS in the Ferret Users Guide.

Ansley

On 8/10/2010 6:04 AM, Marco Steinacher 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

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sjo@xxxxxxxxxxxxx<mailto:sjo@xxxxxxxxxxxxx>.
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