ForestTools (version 1.0.3)

mcws: Marker-Controlled Watershed Segmentation

Description

This function implements the watershed function to segment (i.e.: outline) crowns from a CHM (canopy height model). Segmentation is guided by the point locations of treetops, typically detected using the vwf function. See Meyer & Beucher (1990) for details on watershed segmentation.

Usage

mcws(
  treetops,
  CHM,
  minHeight = 0,
  format = "raster",
  OSGeoPath = NULL,
  IDfield = "treeID"
)

Value

Depending on the setting for format, this function will return a map of outlined crowns as either a SpatRaster class object, in which distinct crowns are given a unique cell value, or a sf class object, in which each crown is represented by a polygon.

Arguments

treetops

sf. The point locations of treetops in sf format.

CHM

SpatRaster. Canopy height model in SpatRaster format. This should be the same CHM that was used to the detect the treetops.

minHeight

numeric. The minimum height value for a CHM pixel to be considered as part of a crown segment. All CHM pixels beneath this value will be masked out. Note that this value should be lower than the minimum height of treetops.

format

string. Format of the function's output. Can be set to either 'raster' or 'polygons'.

OSGeoPath

character. Obsolete. Will be removed next version

IDfield

character. Name of the field for storing the unique tree identifier

Details

Crown segments are returned as either a SpatRaster or a sf (Simple Feature) class object, as defined using the format argument. For many analytic purposes, it is preferable to have crown outlines as polygons. However, polygonal crown maps take up significantly more disk space, and take longer to process. It is advisable to run this function using a raster output first to review results and adjust parameters.

NOTE: when setting format to 'polygons', orphaned segments (i.e.: outlines without an associated treetop) will be removed. This will NOT occur using 'raster' format. This issue will be resolved eventually but requires the watershed function to be rewritten.

References

Meyer, F., & Beucher, S. (1990). Morphological segmentation. Journal of visual communication and image representation, 1(1), 21-46.

See Also

vwf

Examples

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

chm <- rast(kootenayCHM)

# Use variable window filter to detect treetops
ttops <- vwf(chm, winFun = function(x){x * 0.06 + 0.5}, minHeight = 2)

# Segment tree crowns
segs <- mcws(ttops, chm, minHeight = 1)
}