7.6 Ellipsoidal Coordinates

"POINT(50 50.1)" |> st_as_sfc(crs = "OGC:CRS84") -> pt

new versus old methods

image code

par(mfrow = c(1, 2))
par(mar = c(2.1, 2.1, 1.2, .5))
ortho <- st_crs("+proj=ortho +lon_0=50 +lat_0=45")
pol |> st_transform(ortho) |> plot(axes = TRUE, graticule = TRUE, 
                                   main = 's2geometry')
pt |> st_transform(ortho) |> plot(add = TRUE, pch = 16, col = 'red')
# second plot:
plot(pol, axes = TRUE, graticule = TRUE, main = 'GEOS')
plot(pt, add = TRUE, pch = 16, col = 'red')

By default, sf uses geometrical operations from the s2geometry library, interfaced through the s2 package

"POLYGON((40 40, 60 40, 60 50, 40 50, 40 40))" |>
  st_as_sfc(crs = "OGC:CRS84") -> pol
st_intersects(pt, pol) #TRUE

## Sparse geometry binary predicate list of length 1, where the predicate
## was `intersects'
##  1: 1

If one wants sf to use ellipsoidal coordinates as if they are Cartesian coordinates, the use of s2 can be switched off

old <- sf_use_s2(FALSE)
# Spherical geometry (s2) switched off
st_intersects(pol, pt)
# although coordinates are longitude/latitude, st_intersects assumes
# that they are planar
# Sparse geometry binary predicate list of length 1, where the
# predicate was `intersects'
#  1: (empty)
sf_use_s2(old) # restore
# Spherical geometry (s2) switched on