R/poly5_di.R

In timbeR: Calculate Wood Volumes from Taper Functions

#' Estimate the diameter at a given height based on a 5th degree polynomial function.
#'
#' Estimates the diameter at a given height of a tree from the diameter at breast height, total height and the coefficients of the 5th degree polynomial function that describes the tree's taper.
#'
#' @param dbh tree diameter at breast height, in centimeters.
#' @param h total tree height, in meters.
#' @param hi height at which the diameter will be calculated, in meters.
#' @param coef numerical vector containing six coefficients of the 5th degree polynomial function that describes the tree's taper.
#'
#' @return a numeric value indicating the diameter at the given height.
#'
#' @examples
#'
#' library(dplyr)
#' library(minpack.lm)
#' library(timbeR)
#'
#' tree_scaling <- tree_scaling %>%
#' mutate(did = di/dbh,
#'        hih = hi/h)
#'
#' poli5 <- lm(did~hih+I(hih^2)+I(hih^3)+I(hih^4)+I(hih^5),tree_scaling)
#'
#' coef_poli <- coef(poli5)
#'
#' dbh <- 25
#' h <- 20
#' di <- 5
#'
#' poly5_di(dbh, h, di, coef_poli)
#'
#' @export
poly5_di <- function(dbh, h, hi, coef){
  b0 <- coef[[1]]; b1 <- coef[[2]]; b2 <- coef[[3]]; b3 <- coef[[4]]; b4 <- coef[[5]]; b5 <- coef[[6]]
  hih <- hi/h
  (b0+b1*hih+b2*hih^2+b3*hih^3+b4*hih^4+b5*hih^5)*dbh
}