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#' Initial lambda value of Whittaker smoother
#'
#' This function is only suitable for 16-day EVI time-series.
#'
#' @param y Numeric vector
#'
#' @keywords internal
#' @examples
#' library(phenofit)
#' data("MOD13A1")
#'
#' dt <- tidy_MOD13(MOD13A1$dt)
#' st <- MOD13A1$st
#'
#' sitename <- dt$site[1]
#' d <- dt[site == sitename, ] # get the first site data
#' lambda <- init_lambda(d$y)
#' @export
init_lambda <- function(y){
y <- y[!is.na(y)] #rm NA values
mean <- mean(y)
sd <- sd(y)
cv <- sd/mean
skewness <- skewness(y, type = 2)
kurtosis <- kurtosis(y, type = 2)
# lambda was transformed by log10
# lambda <- 0.555484 + 1.671514*mean - 3.434064*sd - 0.052609*skewness + 0.009057*kurtosis
# lambda <- 0.555465 + 1.501239*mean - 3.204295*sd - 0.031902*skewness # Just three year result
# lambda <- 0.831120 + 1.599970*mean - 4.094027*sd - 0.035160*cv - 0.063533*skewness # all year
lambda <- 0.9809 + 0.7247*mean - 2.6752*sd - 0.3854*skewness - 0.0604*kurtosis
# lambda <- 0.817783 + 1.803588*mean - 4.263469*sd - 0.038240*cv - 0.066914*skewness - 0.011289*kurtosis #3y
return(10^lambda)
}
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