CALL SEQUENCE...
      spdlat  SPFILE LATDER OUTPUT
 
 
      DESCRIPTION...
      spdlat  - SPECTRAL LATITUDE DERIVATIVE OF A SPECTRAL ARRAY
 
 
      AUTHOR  - B.Dugas, RPN, FEB 09/94.
      LAST REVISION: $Header: /home/cvs/RMNLIB/utils/rdiag/lspgm/spdlat.ptn,v 4.8 2014/09/25 20:20:53 dugas Exp $
 
 
      PURPOSE - READS A FILE OF SPECTRAL COEFFICIENTS (SPFILE),
                CONVERTS THEM TO THE LATITUDE DERIVATIVE BY
                APPLYING THE ANALYTIC FORMULA...
 
                        D P(N,M)
               COS(LAT) ------- = - N    B(N+1,M) P(N+1,M) +
                         D LAT     (N+1) B(N,  M) P(N-1,M)
 
                                    /  N**2 - M**2 \
                WHERE B(N,M) = SQRT( -------------- ) .
                                    \   4N**2 - 1  /  
 
                (AFTER S.KUBOTA, 1960)
 
      NOTES   - 1) THE RESULTING ARRAYS HAVE ONE EXTRA DEGREES OF FREEDOM
                   IN THE N DIRECTION WHEN THIS FORMULA IS USED. THE ARRAYS
                   ARE THUS LARGER BY ONE ROW (N ==> N+1).
                2) LATDER WILL ACTUALLY BE THE LATITUDE DERIVATIVE 
                   MULTIPLIED BY COS(LAT)
 
 
      INPUT FILE...
       SPFILE = INPUT FILE OF SPECTRAL COEFFICIENTS
 
      OUTPUT FILE...
       LATDER = OUTPUT FILE OF SPECTRAL LATITUDINALLY DERIVED ARRAYS
 
 
 
      EXIT CONDITIONS...
        0 NO PROBLEMS
        1 PREMATURE EOF ON SPFILE
        2 INTERNAL SPECTRAL REPRESENTATION ERROR
        3 LM GREATER THAN MAXM (999)
        4 ERROR READING SPFILE. CHANGING SIZE