unexpected multiple polygons from st_buffer with endCapStyle='FLAT'

[mbedward]

Using sf 1.0-21, I have a pipeline that involves buffering linestrings with a constant buffer distance and endCapStyle='FLAT'. I've noticed that when a line is short relative to the buffer distance, additional disjunct polygons are sometimes generated. For example...

v <- matrix(c(
  245184.6, 6045650,
  245193.3, 6045649,
  245201.7, 6045651,
  245204.3, 6045653
), ncol=2, byrow=TRUE)

line <- st_sfc(st_linestring(v), crs=28356)

buf <- st_buffer(line, dist=50, endCapStyle = 'FLAT')

plot(buf, col="grey90")
plot(line, add=TRUE, lwd=2, col="darkred")

However, if I generate two single-side buffers I get the desired flat end-cap style and the disjunct polygons are not created.

buf_left <- st_buffer(line, dist=50, singleSide = TRUE)
buf_right <- st_buffer(line, dist=-50, singleSide = TRUE)

plot(c(buf_left, buf_right), col="grey90")
plot(line, add=TRUE, lwd=2, col="darkred")

[edzer]

Thanks! These are features of the GEOS software, which sf interfaces, but does not govern.

[kadyb]

You can see what Edzer pointed out in this reprex using direct bindings to GEOS:

library("geos")
geos_version()
#> 3.11.1

line = "LINESTRING (245184.6 6045650, 245193.3 6045649, 245201.7 6045651, 245204.3 6045653)"
line = as_geos_geometry(line)
buf = geos_buffer(line, distance = 50, geos_buffer_params(end_cap_style = "flat"))
plot(buf)

There is some PR here with fix, but I don't know if it will work in this case: libgeos/geos#1323

[mbedward]

Thanks both for the explanation. The work-around using single-sided buffers is performing very well with my data.