Conversion from a SaTScan text file with ellipses to an sf R spatial object

st_from_ellipses.R

st_from_ellipses = function(path, crs = "EPSG:25832", res = 30) {
  e = read.csv(path)
  pnt = sf::st_as_sf(e, coords = c("X", "Y"), crs = crs)
  major = pnt$Major
  minor = pnt$Minor
  angle = pnt$Angle1
  rotation = function(a){
    r = a * pi / 180 #degrees to radians
    matrix(c(cos(r), sin(r), -sin(r), cos(r)), nrow = 2, ncol = 2)
  } 
  coords = sf::st_coordinates(pnt)
  result = list()
  for (i in 1:nrow(coords)) {
    theta = seq(0, 2 * pi, length = res)
    x = coords[i, 1] - minor[i]/2 * cos(theta)
    y = coords[i, 2] - major[i]/2 * sin(theta)
    elp = cbind(x[!is.na(x)], y[!is.na(y)])
    npnt = sf::st_multipoint(elp)
    pol = sf::st_cast(npnt, "LINESTRING")
    pol = (sf::st_sfc(pol) - sf::st_geometry(pnt)[i]) *
      rotation(angle[i]) + sf::st_geometry(pnt)[i]
    result[[i]] = pol
  }
  result = do.call(c, result)
  st_crs(result) = crs
  result = cbind(sf::st_sf(geom = result), sf::st_drop_geometry(pnt))
  result
}