Perform power transformation simultaneously for a data.frame of
raw tree-ring width series.
Usage
powt(rwl)
Arguments
rwl
a data.frame of raw tree-ring widths series, such as that
produced by read.rwl or read.fh
Value
A data.frame containing the power transformed ring width series.
Details
This procedure is a variance stabilization technique implemented after
Cook & Peters (1997): for each series a linear model is fitted on the
logs of level and spread, where level is defined as the local mean
$M_t = \left(R_t + R_{t-1}\right)/2$ with
ring widths R, and spread S is the local standard deviation defined as
$S_t = \left|R_t - R_{t-1}\right|$. The
regression coefficient b from $\log S = k + b \log M$ is then used for the power transform $\star{R}_t =
R_t^{1-b}$.
References
Edward R. Cook and Kenneth Peters (1997) Calculating unbiased
tree-ring indices for the study of climatic and environmental
change. The Holocene 7:361-370.