SUBROUTINE slFTXY (ITYPE,NP,XYE,XYM,COEFFS,J)
- - - - - -
F T X Y
- - - - - -
Fit a linear model to relate two sets of [X,Y] coordinates.
Given:
ITYPE i type of model: 4 or 6 (note 1)
NP i number of samples (note 2)
XYE d(2,np) expected [X,Y] for each sample
XYM d(2,np) measured [X,Y] for each sample
Returned:
COEFFS d(6) coefficients of model (note 3)
J i status: 0 = OK
-1 = illegal ITYPE
-2 = insufficient data
-3 = singular solution
Notes:
1) ITYPE, which must be either 4 or 6, selects the type of model
fitted. Both allowed ITYPE values produce a model COEFFS which
consists of six coefficients, namely the zero points and, for
each of XE and YE, the coefficient of XM and YM. For ITYPE=6,
all six coefficients are independent, modelling squash and shear
as well as origin, scale, and orientation. However, ITYPE=4
selects the "solid body rotation" option; the model COEFFS
still consists of the same six coefficients, but now two of
them are used twice (appropriately signed). Origin, scale
and orientation are still modelled, but not squash or shear -
the units of X and Y have to be the same.
2) For NC=4, NP must be at least 2. For NC=6, NP must be at
least 3.
3) The model is returned in the array COEFFS. Naming the
elements of COEFFS as follows:
COEFFS(1) = A
COEFFS(2) = B
COEFFS(3) = C
COEFFS(4) = D
COEFFS(5) = E
COEFFS(6) = F
the model is:
XE = A + B*XM + C*YM
YE = D + E*XM + F*YM
For the "solid body rotation" option (ITYPE=4), the
magnitudes of B and F, and of C and E, are equal. The
signs of these coefficients depend on whether there is a
sign reversal between XE,YE and XM,YM; fits are performed
with and without a sign reversal and the best one chosen.
4) Error status values J=-1 and -2 leave COEFFS unchanged;
if J=-3 COEFFS may have been changed.
See also slPXY, slINVF, slXYXY, slDCMF
Called: slDMAT, slDMXV
P.T.Wallace Starlink 11 February 1991
Copyright (C) 1995 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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