dplR (version 1.7.6)

powt: Power Transformation of Tree-Ring Data

Description

Perform power transformation for raw tree-ring width.

Usage

powt(rwl, rescale = FALSE)
powt.series(series, rescale = FALSE)

Value

Either 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 or in the case of a single series, a possibly named vector of the same. With rwl, the series IDs are the column names and the years are the row names.

Arguments

rwl

a data.frame of raw tree-ring widths series, such as that produced by read.rwl or read.fh

series

a numeric vector, usually a tree-ring series.

rescale

logical flag. If TRUE then each transformed series is rescaled to have the orginal mean and standard deviation of the input data.

Author

Christian Zang. Patched and improved by Mikko Korpela.

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 \(\star{R}_t = R_t^{1-b}\).

The rescale argument rescales the data to more closely follow the convention in ARSTAN.

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), 361–370.

See Also

rcs

Examples

library(utils)
# many series at once
data(gp.rwl)
gp.pt <- powt(gp.rwl)
hist(summary(gp.rwl)$skew)
hist(summary(gp.pt)$skew)

# single series
gp01A <- gp.rwl[, "01A"]
names(gp01A) <- rownames(gp.rwl)
gp01A.pt <- powt.series(gp01A,rescale=TRUE)
plot(gp01A.pt,gp01A)
abline(c(0,1))