.help xy2xy Jun99 "Slalib Package"
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      SUBROUTINE slXYXY (X1,Y1,COEFFS,X2,Y2)

     - - - - - -
      X Y X Y
     - - - - - -

  Transform one [X,Y] into another using a linear model of the type
  produced by the slFTXY routine.

  Given:
     X1       d        x-coordinate
     Y1       d        y-coordinate
     COEFFS  d(6)      transformation coefficients (see note)

  Returned:
     X2       d        x-coordinate
     Y2       d        y-coordinate

  The model relates two sets of [X,Y] coordinates as follows.
  Naming the elements of COEFFS:

     COEFFS(1) = A
     COEFFS(2) = B
     COEFFS(3) = C
     COEFFS(4) = D
     COEFFS(5) = E
     COEFFS(6) = F

  the present routine performs the transformation:

     X2 = A + B*X1 + C*Y1
     Y2 = D + E*X1 + F*Y1

  See also slFTXY, slPXY, slINVF, slDCMF

  P.T.Wallace   Starlink   5 December 1994

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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