[RegCNET] Deciphering OUT_HEAD.CTL

[bixq]

Hi, Maurice:

The answer is in GrADS documentation :), if you go to the pdef page, 
you'll see.

Because you use the Lambert Conformal projection, the high latitude side 
is stretched far away (in a fan shape), GrADS have to do interpolation to
a lat-lon grid, here xdef and ydef are the definitions of this lat-lon
grid (I call it view-grid), actually, if you prepare two GrADS 
descriptor files,

one with

pdef 58 58 lcc 38.89 -77.00 29.00 29.00 30.00 60.00 -77.00 10000. 10000.
xdef  166 linear  -80.65  0.0450
ydef  126 linear   36.11  0.0450

The other with

pdef 58 58 lcc 38.89 -77.00 29.00 29.00 30.00 60.00 -77.00 10000. 10000.
xdef  42  linear  -80.65 0.180
ydef  32  linear   36.11 0.180

And if you focus on the corner of your plots, I am sure you will 
understand what I said.

Regards,
Xunqiang Bi

On Thu, 19 Nov 2009, Maurice.McHugh at noaa.gov wrote:

> Hello everyone,
>
> I'm trying to understand why in OUT_HEAD.CTL the pdef line states my correct grid sizes (nx,ny), but the xdef and ydef lines which should state the same grid size are different.
>
> For example,
> <snip>
> pdef   58   58 lcc   38.89  -77.00   29.00   29.00   30.00   60.00  -77.00  10000.  10000.
> xdef  166 linear  -80.65  0.0450
> ydef  126 linear   36.11  0.0450
> </snip>
>
> In the above case the 58x58 nature of my grid is correctly specified in pdef, but not in xdef nor ydef which give a 166x126 grid.
>
> In order to better understand the file I went ahead converted the data to netCDF using CDO (climate data operators) and plotted markers at the intersection of xlong and xlat from OUT_HEAD.CTL but when plotted they produced a straight line.  I'm unsure whether this is a CDO translation artifact, or if my understanding of the contents of the OUT_HEAD.CTL file is woefully inadequate.
>
> Any insight to correctly extracting usable data from this file would be much appreciated!
>
> Regards,
>
> Maurice
>
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   Dr. Xunqiang Bi         email:bixq at ictp.it
   Earth System Physics Group
   The Abdus Salam ICTP
   Strada Costiera, 11
   P.O. BOX 586, 34100 Trieste, ITALY
   Tel: +39-040-2240302  Fax: +39-040-2240449
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~