movect.f90

 SUBROUTINE movect(FLAT,FLON,TLAT,TLON,CROT,SROT)
!$$$  SUBPROGRAM DOCUMENTATION BLOCK
!
! SUBPROGRAM:  MOVECT     MOVE A VECTOR ALONG A GREAT CIRCLE
!   PRGMMR: IREDELL       ORG: W/NMC23       DATE: 96-04-10
!
! ABSTRACT: THIS SUBPROGRAM PROVIDES THE ROTATION PARAMETERS
!           TO MOVE A VECTOR ALONG A GREAT CIRCLE FROM ONE
!           POSITION TO ANOTHER WHILE CONSERVING ITS ORIENTATION
!           WITH RESPECT TO THE GREAT CIRCLE.  THESE ROTATION
!           PARAMETERS ARE USEFUL FOR VECTOR INTERPOLATION.
!        
! PROGRAM HISTORY LOG:
!   96-04-10  IREDELL
! 1999-04-08  IREDELL  GENERALIZE PRECISION
!
! USAGE:    CALL MOVECT(FLAT,FLON,TLAT,TLON,CROT,SROT)
!
!   INPUT ARGUMENT LIST:
!     FLAT     - REAL LATITUDE IN DEGREES FROM WHICH TO MOVE THE VECTOR
!     FLON     - REAL LONGITUDE IN DEGREES FROM WHICH TO MOVE THE VECTOR
!     TLAT     - REAL LATITUDE IN DEGREES TO WHICH TO MOVE THE VECTOR
!     TLON     - REAL LONGITUDE IN DEGREES TO WHICH TO MOVE THE VECTOR
!
!   OUTPUT ARGUMENT LIST:
!     CROT     - REAL CLOCKWISE VECTOR ROTATION COSINE
!     SROT     - REAL CLOCKWISE VECTOR ROTATION SINE
!                (UTO=CROT*UFROM-SROT*VFROM;
!                 VTO=SROT*UFROM+CROT*VFROM)
!
! ATTRIBUTES:
!   LANGUAGE: FORTRAN 90
!
!$$$
 IMPLICIT NONE
!
 INTEGER,         PARAMETER     :: KD=selected_real_kind(15,45)
!
 REAL,            INTENT(IN   ) :: FLAT, FLON
 REAL,            INTENT(IN   ) :: TLAT, TLON
 REAL,            INTENT(  OUT) :: CROT, SROT
!
 REAL(KIND=kd),   PARAMETER     :: crdlim=0.9999999
 REAL(KIND=kd),   PARAMETER     :: pi=3.14159265358979
 REAL(KIND=kd),   PARAMETER     :: dpr=180./pi
!
 REAL(KIND=kd)                  :: ctlat,stlat,cflat,sflat
 REAL(KIND=kd)                  :: cdlon,sdlon,crd
 REAL(KIND=kd)                  :: srd2rn,str,ctr,sfr,cfr
! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
!  COMPUTE COSINE OF THE RADIAL DISTANCE BETWEEN THE POINTS.
 ctlat=cos(tlat/dpr)
 stlat=sin(tlat/dpr)
 cflat=cos(flat/dpr)
 sflat=sin(flat/dpr)
 cdlon=cos((flon-tlon)/dpr)
 sdlon=sin((flon-tlon)/dpr)
 crd=stlat*sflat+ctlat*cflat*cdlon
! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
!  COMPUTE ROTATIONS AT BOTH POINTS WITH RESPECT TO THE GREAT CIRCLE
!  AND COMBINE THEM TO GIVE THE TOTAL VECTOR ROTATION PARAMETERS.
 IF(abs(crd).LE.crdlim) THEN
   srd2rn=-1/(1-crd**2)
   str=cflat*sdlon
   ctr=cflat*stlat*cdlon-sflat*ctlat
   sfr=ctlat*sdlon
   cfr=ctlat*sflat*cdlon-stlat*cflat
   crot=srd2rn*(ctr*cfr-str*sfr)
   srot=srd2rn*(ctr*sfr+str*cfr)
!  USE A DIFFERENT APPROXIMATION FOR NEARLY COINCIDENT POINTS.
!  MOVING VECTORS TO ANTIPODAL POINTS IS AMBIGUOUS ANYWAY.
 ELSE
   crot=cdlon
   srot=sdlon*stlat
 ENDIF
! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 END SUBROUTINE movect