Centroid(s) of a vector or clumps in a raster

This function locates the centroid of each geometry of a GVector, or the centroid of each "clump" of same-valued cells in an integer/categorical raster (for information on types of GRasters, see vignette("GRasters", package = "fasterRaster")).

To use this function with a GVector, you need the GRASS v.centerpoint addon. To use the function with a GRaster, you need the addon r.centroids. In either case, the function will try to install the respective addon (i.e., you need to have an internet connection). Once installed, a tool will not need to be installed again.

Usage

# S4 method for class 'GVector'
centroids(x, method = NULL)

# S4 method for class 'GRaster'
centroids(x)

Arguments

x

A GVector or GRaster.

method

GVectors: Character or NULL (default): Method used for calculating centroids. The method of calculation depends on whether the input is a points, lines, or polygons GVector. If the value is NULL, then the default method will be chosen, depending on the geometry type of the GVector:

• points:

  • "mean" (default for points): Mean of coordinates.

  • "median": Geometric median; more robust to outliers.

  • "pmedian": Point in x closest to the geometric median.

• lines:

  • "mid" (default for lines): Mid-point on each line; will fall exactly on the line.

  • "mean": Center of gravity of all line segments; may not fall on the line.

  • "median: Geometric median; may not fall on the line.

• polygons:

  • "mean" (default for polygons): Center of gravity (area), calculated using area triangulation.

  • "median": Geometric mean; may not fall inside the polygon.

  • "bmedian": Geometric mean; minimum distance to boundaries; may not fall inside the polygon.

Partial matching is used and case is ignored.

Value

If the input is a GVector, the output will be a "points" GVector. If the input is a GRaster, the output will be a "points" GVector with a table with statistics on each clump. If the input is a GRaster with more than one layer, the output will be a list of GVectors, with one GVector per layer.

See also

terra::centroids(); GRASS addon tools v.centerpoint and r.centroids.

Examples

if (grassStarted()) {

# Setup
library(sf)
library(terra)

### Points, line, and polygon centroids

# Point centroids:
madDypsis <- fastData("madDypsis")
dypsis <- fast(madDypsis)

dypMean <- centroids(dypsis)
dypMedian <- centroids(dypsis, method = "median")
dypPMedian <- centroids(dypsis, method = "pmedian")

plot(dypsis)
plot(dypMean, col = "red", add = TRUE)
plot(dypMedian, col = "green", pch = 2, add = TRUE)
plot(dypPMedian, col = "blue", pch = 3, add = TRUE)
legend("bottomright",
   legend = c("mean", "median", "pmedian"),
   col = c("red", "green", "blue"),
   pch = c(16, 2, 3),
   xpd = NA
)

# Line centroids:
madRivers <- fastData("madRivers")
rivers <- fast(madRivers)

riversMid <- centroids(rivers)
riversMean <- centroids(rivers, method = "mean")
riversMedian <- centroids(rivers, method = "median")

plot(rivers)
plot(riversMid, col = "red", add = TRUE)
plot(riversMean, col = "green", pch = 2, add = TRUE)
plot(riversMedian, col = "blue", pch = 3, add = TRUE)
legend("bottomright",
   legend = c("mid", "mean", "median"),
   col = c("red", "green", "blue"),
   pch = c(16, 2, 3),
   xpd = NA
)

# Polygon centroids:
madCoast4 <- fastData("madCoast4")
coast4 <- fast(madCoast4)

coastMean <- centroids(coast4)
coastMedian <- centroids(coast4, method = "median")
coastBMedian <- centroids(coast4, method = "bmedian")

plot(coast4)
plot(coastMean, col = "red", add = TRUE)
plot(coastMedian, col = "green", pch = 2, add = TRUE)
plot(coastBMedian, col = "blue", pch = 3, add = TRUE)
legend("bottomright",
   legend = c("mean", "median", "bmedian"),
   col = c("red", "green", "blue"),
   pch = c(16, 2, 1),
   xpd = NA
)

### Centroids of integer GRaster "clumps"

# Load elevation raster
madElev <- fastData("madElev")
elev <- fast(madElev)

# Create clumps of similarly-valued cells
clumps <- clump(elev, minDiff = 0.01, minClumpSize = 1000)

# Centroids:
clumpCents <- centroids(clumps)
clumpCents

plot(clumps)
plot(clumpCents, add = TRUE)


}