R/calculate_d.R

In SpatialWorks/spatialsampler: An Implementation of the Centric Systematic Area Sampling (CSAS) and Simple Spatial Sampling Method (S3M) sampling approaches in R

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#
#' Function to calculate the distance \code{d} given an area size (in square
#' kilometres) of a hexagon in a hexagonal spatial grid determining spatial
#' resolution of spatial sampling
#'
#' @param area Area (square kilometres) of a hexagon in a hexagonal grid.
#'     Determines the spatial resolution of the resulting spatial sample.
#' @param geom Spatial geometry to which \code{area} is specified for. Can be
#'     one of two options: \code{"tri"} for a triangle; \code{hex} for a
#'     hexagon.
#'
#' @return A numeric value for length (kilometres) of \code{d}
#'
#' @author Ernest Guevarra based on equations by Mark Myatt
#'
#' @examples
#' # Calculate d for a triangle with an area size of 130 square kilometres
#' calculate_d(area = 130, geom = "tri")
#'
#' # Calculate d for a hexagon with an area size of 260 square kilometres
#' calculate_d(area = 260, geom = "hex")
#'
#' @export
#'
#
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calculate_d <- function(area, geom = c("tri", "hex")) {
  ## Check if geom more than 1 or is missing
  if(length(geom) > 1 | is.null(geom) | missing(geom)) {
    stop("Polygon type needs to be specified. Try again.", call. = TRUE)
  }

  ## Check if geom is "tri"
  if(geom == "tri") {
    d <- sqrt((area * 4) / (tan(30 * (pi / 180)) * 9))
  }

  ## Check if geom is "hex"
  if(geom == "hex") {
    d <- sqrt((area * 2) / (3 * sqrt(3)))
  }

  ## Return output
  return(d)
}