Snap a point to the closest point on a line segment using sf

Question

I would like to snap a point to the closest point on a road segment using sf::st_snap. However, the function seems to return the wrong result, it\'s snapping my poi

Answer 1

I don't know why st_snap returns the start/end point of the linestring. Maybe that is something for an issue at https://github.com/r-spatial/sf.

Here's a workaround that seems to identify the correct point. Note that st_nearest_points was only introduced recently, so you may need to install sf from github.

nrst = st_nearest_points(point1, roads)
new_point2 = st_cast(nrst, "POINT")[2]

mapView(point1, color="red") + st_buffer(point1, dist = cut_dist) + new_point2 + roads

Wrapping this is a function to return a new geometry set with snapped points below a certain max_dist:

library(sf)
library(mapview)

st_snap_points = function(x, y, max_dist = 1000) {

  if (inherits(x, "sf")) n = nrow(x)
  if (inherits(x, "sfc")) n = length(x)

  out = do.call(c,
                lapply(seq(n), function(i) {
                  nrst = st_nearest_points(st_geometry(x)[i], y)
                  nrst_len = st_length(nrst)
                  nrst_mn = which.min(nrst_len)
                  if (as.vector(nrst_len[nrst_mn]) > max_dist) return(st_geometry(x)[i])
                  return(st_cast(nrst[nrst_mn], "POINT")[2])
                })
  )
  return(out)
}

brew = st_transform(breweries, st_crs(trails))

tst = st_snap_points(brew, trails, 500)

mapview(breweries) + mapview(tst, col.regions = "red") + trails