landmarkreg | Language Reference for FDA Library
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landmarkreg(fdobj, ximarks, x0marks=xmeanmarks, WfdPar=fdPar(defbasis), monwrd=F)
fd
to be registered. The
number of rows N
equals the number of observations, and the number of columns NL equals the
number of landmarks. These landmark times must be in the interior of the
interval over which the functions are defined.
ximarks
is used.
T
, the warping function is estimated using a monotone
smoothing methhod; otherwise, a regular smoothing method is used,
which is not guaranteed to give strictly monotonic warping
functions.
It is essential that the location of every landmark be clearly
defined
in each of the curves as well as the template function. If this is not
the case, consider using the continuous registration function
register.fd
.
Although requiring that a monotone smoother be used to estimate the
warping functions is safer, it adds considerably to the computatation
time since monotone smoothing is itself an iterative process. It is
usually better to try an initial registration with this feature to see
if there are any failures of monotonicity. Moreover, monotonicity
failures can usually be cured by increasing the smoothing parameter
defining
WfdPar.
Not much curvature is usually required in the warping functions, so
a rather low power basis, usually B-splines, is suitable for
defining the functional paramter argument
WfdPar.
A registration with a few prominent landmarks is often a good
preliminary to using the more sophisticated but more lengthy process in
register.fd.