R/gs_zonal_raster.r

In gridsample: Tools for Grid-Based Survey Sampling Design

##  Copyright (c) 2016, Flowminder Foundation, Forrest R. Stevens, Dana R.
##  Thomson
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globalVariables(names=c(".SD"))

#' @author Forrest R. Stevens, \email{forrest.stevens@louisville.edu}
#' @title Zonal statistics calculator
#'
#' @description
#' This function calculates zonal statstics across a raster layer, for each
#' polygon in a rasterized polygon layer.
#'
#' @param x Raster* layer. The layer that zonal statistics should be calculated
#'    from.
#' @param z Raster* layer. A rasterized version of the zonal layer.
#' @param stat character. Name of statistic used to calculate a value across
#'    each polygon in \code{z}. ex. "mean", "sum".
#' @param digits numeric. Number of significant digits in zonal statistic
#'    output.
#' @param na.rm logical. Defines whether to remove \code{NA}
#' @param ... Other variables
#'
#' @return Vector of values representing the calculated statistic for each
#'    polygon, sorted by the order of polygons in the polygon layer.
#'
#' @export
gs_zonal_raster <- function(x,
                            z,
                            stat = "mean",
                            digits = 1,
                            na.rm = TRUE, ...) {
  fun <- match.fun(stat)
  vals <- getValues(x)
  zones <- round(getValues(z), digits = digits)
  rDT <- data.table(vals, z=zones)
  setkey(rDT, z)
  rDT[, lapply(.SD, FUN = fun, na.rm = na.rm), by = z]
}