Scale bar breaks appropriate to a map extent

Returns breaks in kilometres, rounded to a 1/2/5 sequence so the bar reads cleanly at any extent.

Usage

gq_scale_breaks(bbox, n = 3, share = 0.35)

Arguments

bbox

A bbox in a projected CRS with metre units. A geographic bbox has degree spans, so the returned breaks would not be kilometres.

n

Number of intervals.

share

Fraction of the frame width the whole bar should occupy.

Value

A numeric vector of length n + 1, starting at 0, in kilometres.

Details

The whole bar is sized to at most share of the frame width — not share per interval. Sizing per interval overruns the frame, at which point tmap reports "not all scale bar breaks could be plotted" and then silently drops every label but the last, which looks like a styling problem rather than a sizing one.

share is a bound, not a target: the step is the largest 1/2/5 value that fits, so the bar is always at or under it. Rounding to the nearest nice value instead overruns by up to 1.39x, which is the failure the bound exists to prevent.

Examples

bb <- sf::st_bbox(
  c(xmin = 1e6, ymin = 9e5, xmax = 1.1e6, ymax = 1e6),
  crs = 3005
)
gq_scale_breaks(bb)
#> [1]  0 10 20 30

# a wider extent steps up to the next round number
wide <- bb
wide[["xmax"]] <- wide[["xmin"]] + 400000
gq_scale_breaks(wide)
#> [1]  0 20 40 60