Error in st_sample() with type = "hexagonal" for unprojected polygons

[dieghernan]

It seems that there is a bug on st_sample when type="hexagonal" is performed over an unprojected polygon:

library(sf)
nc <- st_read(system.file("shape/nc.shp", package = "sf"))[1, ]
nc = st_transform(nc, 4326)
hexsample_4326 = st_sample(nc, size = 50, type = "hexagonal")

This returns this error:

Error in seq_len(nrow(xy)) : 
  argument must be coercible to non-negative integer
In addition: Warning message:
In seq_len(nrow(xy)) : first element used of 'length.out' argument

Whereas this works fine:

hexsample_other = st_sample(st_transform(nc, 3857), size = 50, type = "hexagonal")

I think the issue comes from this line

sf/R/sample.R

Line 101 in b96ffe8

a0 = as.numeric(st_area(st_make_grid(x, n = c(1,1))))

as it returns the area on m2, and then

sf/R/sample.R

Line 113 in b96ffe8

dx = sqrt(a0 / size / (sqrt(3)/2))

comes in meters as well, and finally

sf/R/sample.R

Lines 180 to 186 in b96ffe8

	bb = st_bbox(obj)
	dy = sqrt(3) * dx / 2
	xlim = bb[c("xmin", "xmax")]
	ylim = bb[c("ymin", "ymax")]
	offset = c(x = (pt[1] - xlim[1]) %% dx, y = (pt[2] - ylim[1]) %% (2 * dy))
	x = seq(xlim[1] - dx, xlim[2] + dx, dx) + offset[1]
	y = seq(ylim[1] - 2 * dy, ylim[2] + 2 * dy, dy) + offset[2]

performs the operations mixing longitude and latitude with meters.

[edzer]

Yes, I can see this, but wonder whether this is better than giving an error message telling that users should first convert data to an equidistant rectangular (+proj=eqc) projection to get meaningful regular and hexagonal samples.