C     ****
C     ****     /TERP/QUANTITIES NEEDED TO TRANSFORM SCALAR FIELDS
C     ****       BETWEEN GEOGRAPHIC AND GEOMAGNETIC COORDINATE SYSTEMS.
C     ****       THESE ARE CALCULATED BY:
C     ****       GRDSTM (GEOGRAPHIC TO GEOMAGMETIC) AND
C     ****       GRDSET (GEOMAGNETIC TO GEOGRAPHIC)
C     ****
!     COMMON/TERP/IG(IMAXMP,JMAXM),JG(IMAXMP,JMAXM),WT(4,IMAXMP,JMAXM),
!    1  IM(ZIMXP1,0:ZJMXP1),JM(ZIMXP1,0:ZJMXP1),DIM(ZIMXP1,0:ZJMXP1),
!    2  DJM(ZIMXP1,0:ZJMXP1)
!
! 1/99: old common /TERP/ is now split into two commons, one for ints, 
!       the other for reals. This is to facilitate reading the new mag 
!       data file on either Crays or SGI's. These data are read from 
!       the mag data file in startup.f. See comments re MAGVOL in 
!       startup.f and input.f.
!
      integer :: ig(imaxmp,jmaxm),jg(imaxmp,jmaxm),
     |           im(zimxp1,0:zjmxp1),jm(zimxp1,0:zjmxp1)
      common/terp_int/ ig,jg,im,jm
      integer,parameter ::
     |  nwterp_int=(imaxmp*jmaxm*2)+(zimxp1*(zjmxp1+1)*2)
!
      real :: wt(4,imaxmp,jmaxm),dim(zimxp1,0:zjmxp1),
     |        djm(zimxp1,0:zjmxp1)
      common/terp_real/ wt,dim,djm
      integer,parameter :: 
     |  nwterp_real=(imaxmp*jmaxm*4)+(zimxp1*(zjmxp1+1)*2)
C     ****
C     ****     WHERE:
C     ****       ZIMX=72, ZJMX=36, IMAXM=80, JMAXM=97, ZIMXP1=ZIMX+1,
C     ****         ZJMXP1=ZJMX+1,IMAXMP=IMAXM+1
C     ****       IG AND JG GIVE GEOGRAPHIC GRID ELEMENT CONTAINING
C     ****         EACH EACH GEOMAGNETIC GRID POINT.
C     ****       WT(4) = INTERPOLATION WEIGHTS TO BE APPLIED TO
C     ****         FUNCTION VALUES AT 4 CORNERS OF GEOGRAPHIC GRID
C     ****         ELEMENT (IG,JG)
C     ****       IM AND JM DEFINE GEOMAGNETIC GRID ELEMENT CONTAINING
C     ****         EACH GEOGRAPHIC GRID POINT.
C     ****       DIM AND DJM ARE FRACTIONS IN I AND J DIRECTIONS USED
C     ****         FOR BILINEAR INTERPOLATION IN GEOMAGNETIC GRID
C     ****         ELEMENT.
C     ****
C