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
An object of class c("rwl", "data.frame") containing the power
transformed ring width series with the series in
columns and the years as rows. The series IDs are the
column names and the years are the row names.
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
Cook, E. R. and Peters, K. (1997) Calculating unbiased
tree-ring indices for the study of climatic and environmental
change. The Holocene, 7(3), 361<U+2013>370.