Arrows for a directional map — shift

Summarizes a map from map_pairwise() on a coarser grid, for drawing arrows. Direction follows the summed vectors. By default length does too, so arrows are long where corridors agree on a direction and shrink where opposing flows cancel; length_by = "magnitude" scales them by the mean size of change instead.

Usage

shift_arrows(
  map,
  cells = 25,
  length = NULL,
  length_by = c("net", "magnitude"),
  scale = c("linear", "sqrt"),
  min_quantile = 0,
  min_length = 0
)

Arguments

map: A directional map from map_pairwise().
cells: Grid size, in cells of map.
length: Length of the longest arrow, in map units. Defaults to about 80% of a grid cell.
length_by: "net" (length of the summed vector) or "magnitude" (mean size of change in the grid cell).
scale: "linear" lengths, or "sqrt" to compress the range so a few very large changes do not shrink every other arrow to a dot.
min_quantile: Drop arrows in the weakest cells (below this quantile of magnitude).
min_length: Shortest arrow, as a share of length. With the default of 0, arrow length is proportional to magnitude, so arrows shrink to nothing where nothing changes.

Value

A data.frame with x, y, xend, yend and magnitude, ready for ggplot2::geom_segment() or graphics::arrows().

Examples

# \donttest{
ex <- corridor_example(acc = TRUE)
v <- both_directions(data.frame(from = c("M03", "M08"), to = c("M20", "M25"), value = 0.05))
v$value[3:4] <- -0.05
m <- map_pairwise(v, ex$acc, ex$sites)
a <- shift_arrows(m, cells = 15)
len <- sqrt((a$xend - a$x)^2 + (a$yend - a$y)^2)
a <- a[len > 0.02 * 15 * terra::res(m)[1], ]   # too short to draw, as in plot_shift()
terra::plot(m$value, col = hcl.colors(100, "viridis"))
arrows(a$x, a$y, a$xend, a$yend, length = 0.05)

# }