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Re: virtual variable in LAS


There are two problems.

1) Each variable defined in the file jpl_have.xml must have a 'url=...'

The 'url=...' attribute may be associated at the <lasdata>, <dataset> or
<variable> level.  If no 'url=...' attribute is defined at the variable
level then the variable inherits its url from the next level up -- the
dataset.  In your case, no URL is defined at the lasdata or dataset
level and none is defined for the 'u_v' variable.  The simple fix is to
define your variable like this:

        <u_v units="M/S" name="u v diff"
         <link match="/lasdata/grids/jpl_3d_grid_1997_"/>

2) The instructions in the FAQ are a little old.  Initialization scripts
need to do more than they used to and should be based on
lasxml/server/jnls/std_initialize.jnl  Personally, I would rename your
init script to 'u_v_diff_init.jnl' and fix the reference to it in the
xml.  The contents of your journal file should look like this:

! u_v_diff_init.jnl
! Based on lasxml/server/jnls/std_initialize.jnl
! 11/01

! Open the files we know we need.

set data "/mnt/sde1/datasets/adjoint/Uave"
set data "/mnt/sde1/datasets/adjoint/Vave"

! We must define var$2_grid for use in differencing.
! This grid must reference a file variable: one that actually
! exists in dataset 'dset' as opposed to a virtual variable
! created with an expression involving other variables.
define grid/like=Uave[d=1] var1_grid_

! Any extra work: defining virtual variables, regridding, etc
! should be done at this time.

let/title="u and v Difference"/units="M/S" u_v = Uave[d=1] - Vave[d=2]

----------------- end of Ferret script -----------------------

I hope that this will allow things to work as usual including

-- Jon

Zhangfan Xing wrote:
> Jon and friends,
> I am trying to create virtual variables with LAS, but get this error:
> No 'url' attribute specified for 'var' element at Ferret.pl line 82
> The faq http://ferret.wrc.noaa.gov/Ferret/LAS/FAQ/virtual_variables.html
> was followed.
> My test site is http://eyre.jpl.nasa.gov/try
> Please check for "u v diff" under first dataset "Assimilation (Adjoint)".
> The *.xml and *.jnl files were posted at
> http://eyre.jpl.nasa.gov/download/for-jon/
> Thanks,
> Zhangfan

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