For sampling in a buffer but only keeping points south of centre

sample_to_the_south.R

# from https://twitter.com/RallidaeRule/status/1083102769169514497

library(sf)
options(stringsAsFactors = FALSE)

nc <- st_read(system.file("shape/nc.shp", package="sf"))
pts <- st_centroid(nc[1:3, ]) 
pts <- pts[ , c('NAME')]
pts <- st_transform(pts, 32617)
buff <- st_buffer(pts, 10000)

# need northing of each point appended to buffer
# use as a filter later
buff$center_N <- st_coordinates(pts)[,2]

# make points
sample_points <- lapply(split(buff, 1:nrow(buff)), function(x) {
  # generate 20 points inside the buffer poly
  spts <- st_coordinates(st_sample(x, 20))
  # drop the ones north of center
  spts <- spts[spts[,2] < x$center_N, ]
  # respatialise
  st_as_sf(as.data.frame(spts), coords = c('X', 'Y'), crs = st_crs(x))
  }) 
sample_points <- do.call('rbind', sample_points)

plot(buff[0], reset = F, axes = T)
plot(pts[0], add = T, col = 'red', pch = 19)
plot(sample_points[0], add = T, col = 'blue', pch = 19)
# \o/