virtual variable

[Zhangfan Xing <xing@xxxxxxxxxxxxxxxxxxxx>]

Hi,

I am trying to set up a virtual variable
following instructions in
http://ferret.wrc.noaa.gov/Ferret/LAS/FAQ/virtual_variables.html

However, LAS complains about:

No 'url' attribute specified for 'var' element at Ferret.pl line 82

The URLs are in fact specified in *.xml for real variables.

Any more detailed docs about this topic other than FAQ?

Attached please find relevant part of my *.xml file and ferret script
that defines the virtual variable.

Thanks in advance.

Zhangfan

---------------- part of *.xml -----------------
        <Have_e units="meters" name="Sea Surface Height 1997"
url="file:/mnt/lasdata0/datasets/adjoint/Have.1997.cdf#Have">
         <link match="/lasdata/grids/jpl_2d_grid_1997"/>
        </Have_e>

        <Have_f units="meters" name="Sea Surface Height 1998"
url="file:/mnt/lasdata0/datasets/adjoint/Have.1998.cdf#Have">
         <link match="/lasdata/grids/jpl_2d_grid_1998"/>
        </Have_f>

        <Have_h units="meters" name="diff">
         <link match="/lasdata/grids/jpl_2d_grid_1998"/>
          <properties>
           <ferret>
            <init_script>diff</init_script>
           </ferret>
          </properties>
        </Have_h>

----------------- ferret script ./jnls/diff.jnl -------------

let/title="Difference"/units="maters" Have_h = Have_e - Have_f