ForestTools (version 1.0.3)

vwf: Variable Window Filter

Description

Implements the variable window filter algorithm (Popescu & Wynne, 2004) for detecting treetops from a canopy height model.

Usage

vwf(
  CHM,
  winFun,
  minHeight = NULL,
  warnings = TRUE,
  minWinNeib = "queen",
  IDfield = "treeID",
  resolution_round = 5
)

Value

Simple feature collection of POINT type. The point locations of detected treetops. The object contains two fields in its data table: height is the height of the tree, as extracted from the CHM, and winRadius is the radius of the search window when the treetop was detected. Note that winRadius does not necessarily correspond to the radius of the tree's crown.

Arguments

CHM

SpatRaster. Canopy height model in SpatRaster format.

winFun

function. The function that determines the size of the window at any given location on the canopy. It should take the value of a given CHM pixel as its only argument, and return the desired *radius* of the circular search window when centered on that pixel. Size of the window is in map units.

minHeight

numeric. The minimum height value for a CHM pixel to be considered as a potential treetop. All CHM pixels beneath this value will be masked out.

warnings

logical. If set to FALSE, this function will not emit warnings related to inputs.

minWinNeib

character. Define whether the smallest possible search window (3x3) should use a queen or a rook neighborhood.

IDfield

character. Name of field for unique tree identifier

resolution_round

integer. The raster resolution is used to compute the dimensions of the search windows. By default, this resolution is rounded to 5 decimal places. The number of decimal places can be changed using this parameter. Increasing this value is also a work-around for errors relating to non-square cell sizes.

Details

This function uses the resolution of the raster to figure out how many cells the window needs to cover. This means that the raster value (representing height above ground) and the map unit (represented by the raster's resolution), need to be in the _same unit_. This can cause issues if the raster is in lat/lon, whereby its resolution is in decimal degrees.

References

Popescu, S. C., & Wynne, R. H. (2004). Seeing the trees in the forest. Photogrammetric Engineering & Remote Sensing, 70(5), 589-604.

See Also

mcws

Examples

if (FALSE) {
library(terra)
library(ForestTools)

chm <- rast(kootenayCHM)

# Set function for determining variable window radius
winFunction <- function(x){x * 0.06 + 0.5}

# Set minimum tree height (treetops below this height will not be detected)
minHgt <- 2

# Detect treetops in demo canopy height model
ttops <- vwf(chm, winFunction, minHgt)
}