Power Transformation of Tree-Ring Data
Perform power transformation simultaneously for a data.frame of
raw tree-ring width series.
powt(rwl, rescale = FALSE)
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 = (R_t + R_(t-1))/2 with ring widths R, and spread S is the local standard deviation defined as S_t = |R_t - R_(t-1)|. The regression coefficient b from log S = k + b log M is then used for the power transform R*_t = R_t^(1-b).
The rescale argument rescales the data to more closely follow the
convention in ARSTAN.
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.
Christian Zang. Patched and improved by Mikko Korpela.
Cook, E. R. and Peters, K. (1997) Calculating unbiased tree-ring indices for the study of climatic and environmental change. The Holocene, 7(3), 361–370.
library(utils) data(gp.rwl) gp.pt <- powt(gp.rwl) hist(summary(gp.rwl)$skew) hist(summary(gp.pt)$skew)
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