#' Direction to group centroid
#'
#' \code{direction_to_centroid} calculates the direction of each relocation to
#' the centroid of the spatiotemporal group identified by \code{group_pts}. The
#' function accepts a \code{data.table} with relocation data appended with a
#' \code{group} column from \code{group_pts} and centroid columns from
#' \code{centroid_group}. Relocation data should be in planar coordinates
#' provided in two columns representing the X and Y coordinates.
#'
#' The \code{DT} must be a \code{data.table}. If your data is a
#' \code{data.frame}, you can convert it by reference using
#' \code{\link[data.table:setDT]{data.table::setDT}} or by reassigning using
#' \code{\link[data.table:data.table]{data.table::data.table}}.
#'
#' This function expects a \code{group} column present generated with the
#' \code{group_pts} function and centroid coordinate columns generated with the
#' \code{centroid_group} function. The \code{coords} and \code{group} arguments
#' expect the names of columns in \code{DT} which correspond to the X and Y
#' coordinates and group columns.
#'
#' @inheritParams distance_to_centroid
#' @inheritParams group_pts
#'
#' @return \code{direction_to_centroid} returns the input \code{DT} appended
#' with a \code{direction_centroid} column indicating the direction to group
#' centroid in radians. The direction is measured in radians in the range
#' of 0 to 2 * pi from the positive x-axis.
#'
#' A message is returned when \code{direction_centroid} column already exist
#' in the input \code{DT}, because they will be overwritten.
#'
#' @export
#' @family Distance functions
#' @seealso [centroid_group], [group_pts]
#' @references
#' See example of using direction to group centroid:
#' * <https://doi.org/10.1016/j.cub.2017.08.004>
#'
#' @examples
#' # Load data.table
#' library(data.table)
#' \dontshow{data.table::setDTthreads(1)}
#'
#' # Read example data
#' DT <- fread(system.file("extdata", "DT.csv", package = "spatsoc"))
#'
#' # Cast the character column to POSIXct
#' DT[, datetime := as.POSIXct(datetime, tz = 'UTC')]
#'
#' # Temporal grouping
#' group_times(DT, datetime = 'datetime', threshold = '20 minutes')
#'
#' # Spatial grouping with timegroup
#' group_pts(DT, threshold = 5, id = 'ID',
#' coords = c('X', 'Y'), timegroup = 'timegroup')
#'
#' # Calculate group centroid
#' centroid_group(DT, coords = c('X', 'Y'), group = 'group', na.rm = TRUE)
#'
#' # Calculate direction to group centroid
#' direction_to_centroid(DT, coords = c('X', 'Y'))
direction_to_centroid <- function(
DT = NULL,
coords = NULL) {
# Due to NSE notes in R CMD check
direction_centroid <- NULL
if (is.null(DT)) {
stop('input DT required')
}
if (length(coords) != 2) {
stop('coords requires a vector of column names for coordinates X and Y')
}
xcol <- data.table::first(coords)
ycol <- data.table::last(coords)
pre <- 'centroid_'
centroid_xcol <- paste0(pre, xcol)
centroid_ycol <- paste0(pre, ycol)
centroid_coords <- c(centroid_xcol, centroid_ycol)
if (any(!(coords %in% colnames(DT)))) {
stop(paste0(
as.character(paste(setdiff(
coords,
colnames(DT)
), collapse = ', ')),
' field(s) provided are not present in input DT'
))
}
if (any(!(DT[, vapply(.SD, is.numeric, TRUE), .SDcols = c(coords)]))) {
stop('coords must be numeric')
}
if (any(!(centroid_coords %in% colnames(DT)
))) {
stop(paste0(
as.character(paste(setdiff(
centroid_coords,
colnames(DT)
), collapse = ', ')),
' field(s) provided are not present in DT, did you run centroid_group?'
))
}
if (any(!(DT[, vapply(.SD, is.numeric, TRUE),
.SDcols = c(centroid_coords)]))) {
stop('centroid coords must be numeric')
}
if ('direction_centroid' %in% colnames(DT)) {
message('direction_centroid column will be overwritten by this function')
data.table::set(DT, j = 'direction_centroid', value = NULL)
}
DT[, direction_centroid := fifelse(
.SD[[xcol]] == .SD[[centroid_xcol]] &
.SD[[ycol]] == .SD[[centroid_ycol]],
units::as_units(NaN, 'rad'),
units::as_units(
atan2(.SD[[centroid_ycol]] - .SD[[ycol]],
(.SD[[centroid_xcol]] - .SD[[xcol]])),
'rad'
)
)]
DT[direction_centroid < units::as_units(0, 'rad'),
direction_centroid := direction_centroid + units::as_units(2 * pi, 'rad')]
return(DT[])
}
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.