[IDEA] Calc terrain covariates

[eva0marques]

In process_gmted() or calc_gmted() (not clear for me where calculations are made since some of them are actually done in process_gmted()), we could add other variables derived from elevation: slope, aspect, flowdir, roughness. It is very simple using terra::terrain() function. See this old code of mine:

#' Add terrain covariates to a terra::SpatRaster (dem included)
#'
#' @param dem_path a path to dem raster
#' @param sp_vect a terra::SpatVector
#' @returns the same terra::SpatVector with terrain covariates
#' (dem, slope, aspect, roughness, flowdir)
#' @export
add_terrain <- function(sp_vect, dem_path) {
  dem_rast <- terra::rast(dem_path)
  names(dem_rast) <- "dem"
  dem_rast$slope <- terra::terrain(dem_rast$dem, "slope")
  # aspect is in degrees, clockwise from North
  # if no slope: 90
  dem_rast$aspect <- terra::terrain(dem_rast$dem, "aspect")
  # Roughness is the difference between the maximum and the
  # minimum value of a cell and its 8 surrounding cells.
  dem_rast$roughness <- terra::terrain(dem_rast$dem, "roughness")
  # Values encoded as power of 2 (x is the cell): IT IS A DISCRETE VARIABLE
  # 32 64 128
  # 16 x   1
  # 8  4  2
  # Cells are set to 0 if no lower neighboring.
  dem_rast$flowdir <- terra::terrain(dem_rast$dem, "flowdir")
  # extract at sp_vect locations
  sp_vect_cov <- terra::project(sp_vect, terra::crs(dem_rast)) %>%
    terra::extract(x = dem_rast, bind = TRUE) %>%
    terra::project(terra::crs(sp_vect))
  if (any(is.na(sp_vect_cov$dem))) {
    warning("NAs found in dem column")
  }
  if (any(is.na(sp_vect_cov$slope))) {
    warning("NAs found in slope column")
  }
  if (any(is.na(sp_vect_cov$aspect))) {
    warning("NAs found in aspect column")
  }
  if (any(is.na(sp_vect_cov$roughness))) {
    warning("NAs found in roughness column")
  }
  if (any(is.na(sp_vect_cov$flowdir))) {
    warning("NAs found in flowdir column")
  }
  return(sp_vect_cov)
}