# NOT RUN {
dist_vincenty = function(p1, p2, a, f) geosphere::distVincentyEllipsoid(p1, p2, a, a * (1-f), f)
line = st_sfc(st_linestring(rbind(c(30,30), c(40,40))), crs = 4326)
st_length(line)
st_length(line, dist_fun = dist_vincenty)
# }
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