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Identify terrain feature types

Source: R/geomorphons.r
geomorphons.Rd

Geomorphons are idealized terrain types calculated from an elevator raster based on a moving window of a given size. The window is a torus (which can have an inner radius of 0, so can also be a circle), which allows it to identify geomorphons of a given size while ignoring ones larger or smaller. There are 10 basic geomorphons. Consult the The GRASS module r.geomorphon for more details and diagrams of each type of geomorphon. They include:

  1. Flat areas: Focal area has approximately the same elevation as surrounding areas

  2. Pits: An area is lower than all other surrounding areas

  3. Valley: Focal area has elevation similar to two opposing side of the window but lower than the other two opposing sides

  4. Footslope: Focal region is at the "bottom" of a slope

  5. Hollow: A small valley/indention in the crest of a hill

  6. Slope: Cells in the window form an approximately uniform slope

  7. Spur: An extrusion at the foot of a hill (i.e.,, a small hill extending out from the foot of a slope)

  8. Shoulder: The crest of a slope

  9. Ridge: Opposite of a valley; focal area is higher than two opposing sides but approximately the same elevation as the other two opposing sides

  10. Peak: Focal area is higher than any other in the window

Usage

# S4 method for class 'GRaster'
geomorphons(
  x,
  inner = 0,
  outer = 3,
  unit = "cells",
  flat = 1,
  flatDist = 0,
  mode = "1"
)

Arguments

x

A single-layer GRaster, typically representing elevation.

inner, outer

Integer: Inner and outer radii of the torus used to identify geomorphons, in cells or meters (set by argument unit). The inner default value is 0 and the outer default value is 3. The outer radius sets the maximum size of a geomorphon that that can be identified, and inner sets the smallest size. If unit is "meters", the value of outer must be larger than the smaller dimension of any cell in the east-west and north-south directions.

unit

Character: Units of inner and outer; can be either "cells" (default) or "meters". Partial matching is used.

flat

Numeric value >= 0: Minimum difference (in degrees) between the focal area areas around it for a geomorphon to be considered as "flat". Larger cells (i.e., ~1 km resolution or larger) require smaller values (<<1) to correctly identify flat areas. Higher values result in more areas being classified as "flat" geomorphons. The default value is 1.

flatDist

Numeric: Distance (in meters) to correct for the effect of large distances on the diminished capacity to identify "flat" geomorphons. If the distance between the focal area and a surrounding area surpasses this distance, then the effective value of flat will be reduced

mode

Character: Method for implementing the zenith/line-of-site search. Partial matching is used:

  • "1" (default): The "original" geomorphon mode (in GRASS module r.geomorphon, the "anglev1" method)

  • "2": Better handling of cases with equal zenith/nadir angles (the "anglev2" method)

  • "2d": As "2", but takes into account zenith/nadir distance ("anglev2_distance" method)

Value

A categorical GRaster where each geomorphon is a category.

See also

Module r.geomorphon in GRASS

Examples

if (grassStarted()) {

# Setup
library(terra)

# Example data
madElev <- fastData("madElev")

# Convert a SpatRaster to a GRaster:
elev <- fast(madElev)

# Geomorphons:
geos <- geomorphons(elev)
geos
levels(geos) # levels
freq(geos) # frequencies

col <- c("gray90", "red", "orange", "blue", "green", "pink", "firebrick",
  "purple", "gray50", "black")
plot(geos, col = col)

}