C
      SUBROUTINE GRDTRP(AM,AG,IM,JM,DIM,DJM,LG,LM,IMAXG,JMAXG,IMAXM,
     1JMAXM)
      implicit none
C     ****
C     ****     SUBROUTINE TRANSFORMS SCALAR FIELD GIVEN ON GEOMAGNETIC
C     ****     GRID TO GEOGRAPHIC GRID USING COEFFICIENTS FOR BILINEAR
C     ****     INTERPOLATION PREVIOUSLY GENERATED BY CALL TO GRDSET.
C     ****
C     ****     GEOGRAPHIC GRID:
C     ****       IMAXG EQUALLY SPACED LONGITUDE POINTS.  RANGE
C     ****         -PI TO PI.
C     ****       JMAXG LATITUDE POINTS, NOT NECESSARILY EQUALLY
C     ****         SPACED.  RANGE -PI/2 TO P/2.
C     ****
C     ****     GEOMAGNETIC GRID:
C     ****       IMAXM EQUALLY SPACED LONGITUDE POINTS.  RANGE
C     ****         -PI TO PI.
C     ****       JMAXM LATITUDE POINTS, NOT NECESSARILY EQUALLY
C     ****         SPACED.  RANGE -PI/2 TO P/2.  MUST INCLUDE
C     ****         POLES
C     ****
C     ****     INPUT:
C     ****
C     ****       AM(LM,1) IS 2-DIM SCALAR FIELD GIVEN ON GEOMAGNETIC
C     ****         LATITUDE/LONGITUDE GRID.
C     ****         NB.  PERIODIC POINT IS REPEATED SO THAT
C     ****         AM(IMAXM+1,JM) = AM(1,JM)
C     ****         (LM.GE.IMAXM+1)
C     ****
C     ****       IMAXG, JMAXG  DIMENSIONS OF GEOGRAPHIC GRID
C     ****
C     ****       IMAXM, JMAXM  DIMENSIONS OF GEOMAGNETIC GRID
C     ****
C     ****       LM IS FIRST DIMENSION OF ARRAY AM INCALLING PROGRAM
C     ****         (LM.GE.IMAXM+1)
C     ****
C     ****       IM(LG,1) GIVES ROUNDED DOWN GEOMAGNETIC LONGITUDE
C     ****         INDEX FOR EACH GEOGRAPHIC GRID POINT
C     ****
C     ****       JM(LG,1) GIVES ROUNDED DOWN GEOMAGNETIC LATITUDE
C     ****         INDEX FOR EACH GEOGRAPHIC GRID POINT
C     ****
C     ****       DIM(LG,1) IS FRACTIONAL GEOMAGNETIC LONGITUDE INDEX
C     ****         FOR EACH GEOGRAPHIC GRID POINT.  USED FOR BILINEAR
C     ****         INTERPOLATION WITHIN GRID CELL.
C     ****
C     ****       DJM(LG,1) IS FRACTIONAL GEOMAGNETIC LATITUDE INDEX
C     ****         FOR EACH GEOGRAPHIC GRID POINT.  USED FOR BILINEAR
C     ****         INTERPOLATION WITHIN GRID CELL.
C     ****
C     ****       LG IS FIRST DIMENSION OF ARRAYS AM,IM,JM,DIM,DJM IN
C     ****         CALLING PROGRAM.
C     ****         (LG.GE.IMAXG)
C     ****
C     ****       NOTE:  IM, JM, DIM, DJM ARE PRODUCED BY PREVIOUS CALL
C     ****         TO GRDSET.
C     ****
C     ****     OUTPUT:
C     ****
C     ****       AG(LG,1) IS 2-DIM SCALAR FIELD TRANSFORMED TO
C     ****         GEOGRAPHIC LATITUDE/LONGITUDE GRID.
C     ****         (LG.GE.IMAXG)
C     ****
C     ****     PERFORM BILINEAR INTERPOLATION
C     ****
!
! Args:
      integer,intent(in) :: lg,lm,imaxg,jmaxg,imaxm,jmaxm,
     |  IM(LG,1),JM(LG,1)
      real,intent(in) :: AM(LM,1),DIM(LG,1),DJM(LG,1)
      real,intent(out) :: AG(LG,1)
!
! Local:
      integer :: jg,ig
!
      DO 1 JG = 1,JMAXG
        DO 1 IG = 1,IMAXG
          AG(IG,JG) =
     1    AM(IM(IG,JG),JM(IG,JG))*(1.-DIM(IG,JG))*(1.-DJM(IG,JG))+
     2    AM(IM(IG,JG)+1,JM(IG,JG))*DIM(IG,JG)*(1.-DJM(IG,JG))+
     3    AM(IM(IG,JG),JM(IG,JG)+1)*(1.-DIM(IG,JG))*DJM(IG,JG)+
     2    AM(IM(IG,JG)+1,JM(IG,JG)+1)*DIM(IG,JG)*DJM(IG,JG)
    1 CONTINUE
      RETURN
      END
C