SUBROUTINE  KONSC (Z0,XF,YF,IA,IB,JA,JB,ZE,NX,NY,PL)

C     [SINGLE CONTOUR]
C     A GENERAL PURPOSE SUBROUTINE WHICH MAY BE USED TO GENERATE SIMPLE
C     CONTOURS, OR CONTOURS OF ORTHOGRAPHIC RELIEF.
C     Z0        CONTOUR LEVEL SOUGHT
C     (XF,YF)   LIGHTING DIRECTION FOR ORTHOGRAPHIC RELIEF
C     (IA,IB)   X-INTERVAL TO BE CONTOURED
C     (JA,JB)   Y-INTERVAL TO BE CONTOURED
C     ZE(NX,NY) ARRAY OF FUNCTION VALUES
C     PL        PEN MOVEMENT SUBROUTINE
C     [06-JAN-75]

      LOGICAL*1     FE(35,35,2)
      DIMENSION   ZE(1)
      COMMON/KON/ I1,J1,I2,J2,I3,J3,Z1,Z2,Z3

      U(I,J)=ZE(I+NX*(J-1))-Z0+XF*FLOAT(I-1)+YF*FLOAT(J-1)
      ZP(I1,I2)=FLOAT(I1-1)-Z1*(FLOAT(I2-I1)/(Z2-Z1))

      IF ((IB-IA).GT.35) RETURN
      IF ((JB-JA).GT.35) RETURN
      XS=1.0/FLOAT(NX-1)
      YS=1.0/FLOAT(NY-1)
      II=MAX0(IA,IB-1)
      JJ=MAX0(JA,JB-1)
      DO 10 I=IA,II
      DO 10 J=JA,JJ
      Z11=U(I,J)
      Z12=U(I,J+1)
      Z21=U(I+1,J)
      Z22=U(I+1,J+1)
      ZP1=AMAX1(Z11,Z12,Z21)
      ZM1=AMIN1(Z11,Z12,Z21)
      ZP2=AMAX1(Z12,Z21,Z22)
      ZM2=AMIN1(Z12,Z21,Z22)
      FE(I-IA+1,J-JA+1,1)=(ZP1.LT.0.0).OR.(ZM1.GT.0.0)
   10 FE(I-IA+1,J-JA+1,2)=(ZP2.LT.0.0).OR.(ZM2.GT.0.0)
      DO 40 K=1,2
      DO 40 I=IA,II
      DO 40 J=JA,JJ
      IF (FE(I-IA+1,J-JA+1,K)) GO TO 40
      CALL KONIT (I,J,K)
      Z1=U(I1,J1)
      Z2=U(I2,J2)
      Z3=U(I3,J3)
      IF (SIGN(1.0,Z1).EQ.SIGN(1.0,Z2)) CALL KONXV (1,3)
      IF (SIGN(1.0,Z1).EQ.SIGN(1.0,Z3)) CALL KONXV (1,2)
      CALL KONSA
      DO 30 L=1,2
      CALL PL (XS*ZP(I1,I2),YS*ZP(J1,J2),.FALSE.)
   20 CALL KONNC
      CALL PL (XS*ZP(I1,I2),YS*ZP(J1,J2),.TRUE.)
      I0=MIN0(I1,I2,I3)-IA+1
      J0=MIN0(J1,J2,J3)-JA+1
      K0=MOD(I1+I2+I3,3)
      IF (FE(I0,J0,K0)) GO TO 30
      FE(I0,J0,K0)=.TRUE.
      IF ((I3.LT.IA).OR.(I3.GT.IB).OR.(J3.LT.JA).OR.(J3.GT.JB)) GO TO 30
      Z3=U(I3,J3)
      GO TO 20
   30 CALL KONRE
   40 CONTINUE
      RETURN
      END