Problem with missing values

[Simon Evans <simon@xxxxxxxxxxxxxxxxx>]

    Dear Ferret users,

    When setting up a NetCDF file for ocean model (POM) output, I try
 to assign a special value to land cells by setting the _FillValue or
 missing_value attribute.

    For example, in the case of the variable EL (sea surface elevation):

        call NCAPTC(CDFID,ELID,'long_name',NCCHAR,9,'Elevation',IRET)
        call NCAPTC(CDFID,ELID,'units',NCCHAR,6,'meters',IRET)
  --->  call NCAPT(CDFID,ELID,'missing_value',NCDOUBLE,1,SPVAL,IRET)

 where SPVAL is -999.0 or some such number. Then when writing the
 variable to the CDF file, the code fragment is:

        do j = 1,JM
         do i = 1,IM
           if (FSM(i,j).eq.0.0) then
             TMP2D(i,j) = SPVAL      ! Land cell identifier
           else
             TMP2D(i,j) = EL(i,j)
           endif
          enddo
        enddo

        ELID = NCVID(CDFID,'EL',IRET)
        call NCVPT(CDFID,ELID,CORNER,EDGES,TMP2D,IRET)

  where FSM is a mask which is zero on land cells and unity everywhere
 else.

     The trouble is that while an older version of Ferret (v4.11) seems
 to recognise the missing value flag, the newest version avaliable to
 me (v4.90) does not. For example, with the earlier version, I get
 the missing value indicator ("...") as I should:

>   yes? list/j=2/i=1:5 EL
>                Elevation (meters)
>                T (days): 0
>                DATA SET: ./spiral.cdf
>                2     
>                2
>    1   / 1:    ....
>    2   / 2:  0.0000
>    3   / 3:  0.0000
>    4   / 4:  0.0000
>    5   / 5:  0.0000

   while with the later version I get something strange:

>   yes? list/j=2/i=1:5 EL
>                Elevation (meters)
>                T (days): 0
>                DATA SET: ./spiral.cdf
>                2    
>                2
>    1   / 1: -999.0
>    2   / 2:   ....
>    3   / 3:   ....
>    4   / 4:   ....
>    5   / 5:   ....

   Ferret v4.90 seems to interpret the zeros as missing values, but 
 fails to correctly interpret my intended missing-value flag!

   Any ideas as to what is happening here?