#' @title Surface Relief Ratio
#' @description Calculates the Pike (1971) Surface Relief Ratio
#'
#' @param x A terra SpatRaster object
#' @param s Focal window size
#' @param ... Additional arguments passed to terra::lapp
#'
#' @details
#' Describes rugosity in continuous raster surface within a specified window.
#' The implementation of SRR can be shown as: (mean(x) - min(x)) / (max(x) - min(x))
#'
#' @return A terra SpatRaster object of Pike's (1971) Surface Relief Ratio
#'
#' @author Jeffrey S. Evans <jeffrey_evans@@tnc.org>
#'
#' @examples
#' \donttest{
#' library(terra)
#' elev <- rast(system.file("extdata/elev.tif", package="spatialEco"))
#' r.srr <- srr(elev, s=5)
#' plot(r.srr, main="Surface Relief Ratio")
#' }
#' @export srr
srr <- function(x, s = 5, ...) {
if (!inherits(x, "SpatRaster"))
stop(deparse(substitute(x)), " must be a terra SpatRaster object")
m <- matrix(1, nrow=s, ncol=s)
rmin <- terra::focal(x, w=m, fun=min)
rmax <- terra::focal(x, w=m, fun=max)
rmean <- terra::focal(x, w=m, fun=mean)
return( terra::lapp(c(rmean,rmin,rmax),
fun=function(x,y,z) { (x - y ) / ( z - y ) }, ... ) )
}
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