Comparing Two Curvilinear Variables

[Daryl Herzmann <akrherz@xxxxxxxxxxx>]

Hi Ferret!

I am in a dill of a pickle and have been trying to think like a Ferret 
without much luck.  Perhaps a kind person could help me out with a 
documentation reference I have probably missed :(

I have MM5 model forecasted precip on one curvilinear grid.  The NetCDF 
highlights are:

    float rain_non(time, i_cross, j_cross) ;
        rain_non:long_name = "ACCUMULATED NONCONVECTIVE PRECIPITATION" ;
        rain_non:units = "cm" ;
    float latitcrs(i_cross, j_cross) ;
        latitcrs:long_name = "LATITUDE (SOUTH NEGATIVE)" ;
        latitcrs:units = "degrees" ;
    float longicrs(i_cross, j_cross) ;
        longicrs:long_name = "LONGITUDE (WEST NEGATIVE)" ;
        longicrs:units = "degrees" ;

And I have NCEP stage4 precip data on a different curvilinear grid. Again, 
the NetCDF highlights.

    float APCP_SFC(YCells, XCells) ;
        APCP_SFC:long_name = "Total precipitation" ;
        APCP_SFC:units = "inch" ;
    float longitude(YCells, XCells) ;
        longitude:long_name = "longitude" ;
        longitude:units = "degrees_east" ;
    float latitude(YCells, XCells) ;
        latitude:long_name = "latitude" ;
        latitude:units = "degrees_north" ;


So, thinking like a Ferret, I would think I need to regrid these two 
curvilinear grids onto a common regular lat-lon grid and then proceed with 
the comparison between the two.  Unfortunately, I can't figure out how to 
do this :(  From the docs, I see this in Chp 8 Sec 8.2

"To perform the analysis in a rectilinear coordinate system, the
conversion of the curvilinear data is most easily done with SAMPLEXY_CURV
(under development06/00 )."

I can't seem to find that samplexy_curve was implemented. So I searched 
the email archives and found this post which suggests to use scat2grid 
functions.

http://ferret.pmel.noaa.gov/Ferret/Mail_Archives/fu_2001/msg00084.html

But those functions don't seem to work for me.  The resulting grid is 
always "No valid data".  For example I did:

yes? use APCP_06171200.nc  ! My NCEP data
yes? DEFINE AXIS/X=100w:90w:0.5/UNITS=degrees x0
yes? DEFINE AXIS/Y=40n:50n:0.5/UNITS=degrees y0
yes? show data
     currently SET data sets:
    1> ./APCP_06171200.nc  (default)
 name     title                             I         J         K      L
 APCP_SFC Total precipitation              1:1121    1:881     ...    ...
 MAPPROJECTION
          MapProjection                    ...       ...       ...    ...
 LONGITUDE
          longitude                        1:1121    1:881     ...    ...
 LATITUDE latitude                         1:1121    1:881     ...    ...

yes? LET outg = SCAT2GRIDGAUSS_XY(longitude, latitude, APCP_SFC, x[gx=x0], 
y[gy=y0],0.5,0.5,2,2)
yes? shade outg


I am using Ferret 5.53 on Fedora Core 1 and am probably doing something 
ignorant :(

thanks,
  daryl