ExtractAlongTransect: Extract Raster Values Along Transect Line

Description

This function extracts values from raster layer(s) along a user defined transect line.

Usage

ExtractAlongTransect(transect, r)

Arguments

transect

'SpatialPoints' or 'SpatialLines'. Transect line or its vertices.

'RasterLayer', 'RasterStack', or 'RasterBrick'. Raster layer(s)

Value

A 'list' is returned with components of class 'SpatialPointsDataFrame'. These components represent continuous piecewise line segments along the transect. The following variables are specified for each coordinate point in the line segment:

dist: distance along the transect line.
2, …, n: extracted value for each raster layer in r, where column names match their respective raster layer name.

Details

The transect line is described using a simple polygonal chain. The transect line and raster layer(s) must be specified in a coordinate reference system.

Examples

Run this code

# NOT RUN {
library(raster)

coords <- rbind(c(-100, -90), c(80, 90), c(80, 0), c(40, -40))
crs <- CRS("+init=epsg:4326")
transect <- SpatialPoints(coords, proj4string = crs)
r <- raster(nrows = 10, ncols = 10, ymn = -80, ymx = 80, crs = crs)
names(r) <- "value"
set.seed(0)
r[] <- runif(ncell(r))
r[4, 6] <- NA
plot(r, xlab = "x", ylab = "y")
lines(SpatialLines(list(Lines(list(Line(coords)), ID = "Transect")), proj4string = crs))
points(transect, pch = 21, bg = "red")
segs <- ExtractAlongTransect(transect, r)
for (i in 1:length(segs)) points(segs[[i]], col = "blue")

dev.new()
xlab <- "Distance along transect"
ylab <- "Raster value"
xlim <- range(vapply(segs, function(seg) range(seg@data[, "dist"]), c(0, 0)))
ylim <- range(vapply(segs, function(seg) range(seg@data[, "value"], na.rm = TRUE),
                     c(0, 0)))
plot(NA, type = "n", xlab = xlab, ylab = ylab, xlim = xlim, ylim = ylim)
for (i in 1:length(segs))
  lines(segs[[i]]@data[, c("dist", "value")], col = rainbow(length(segs))[i])
coords <- coordinates(transect)
n <- length(transect)
d <- cumsum(c(0, as.matrix(dist((coords)))[cbind(1:(n - 1), 2:n)]))
abline(v = d, col = "grey", lty = 2)
mtext(paste0("(", paste(head(coords, 1), collapse = ", "), ")"), adj = 0)
mtext(paste0("(", paste(tail(coords, 1), collapse = ", "), ")"), adj = 1)

# }