SUBROUTINE LINEQ(X1,X2,Y1,Y2,A,B,IER)  5
***subroutine     LINEQ
*
*auteur           Louis Garand  - rpn - dorval
*
*REVISION 001     JACQUES HALLE - DDO - DORVAL - 421-4660
*                                 Decembre 1995
*                 Generaliser pour toutes les plateformes satellitaires.
*
*langage          fortran 5
*
*objet            calculate slope and intercept of a line.
*
*appel            CALL  LINEQ(X1,X2,Y1,Y2,A,B,IER)
*
*arguments        x1  - input  - coordinate x of point 1
*                 x2  - input  - coordinate x of point 2
*                 y1  - input  - coordinate y of point 1
*                 y2  - input  - coordinate y of point 2
*                 a   - output - slope
*                 b   - output - intercept
*                 ier - output - error code (0=ok)
**
      implicit none
c
      integer  ier
c
      real  x1, x2, y1, y2, a, b
c
      ier = 0
c
      IF((X2-X1).EQ.0.)THEN 
         IER=-1
         RETURN
      ENDIF 
c
      A=(Y2-Y1)/(X2-X1) 
      B=Y1-A*X1 
c
      RETURN
      END