Find points close to an outline. Help needed!

close-points.R

  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # A bunch of points
  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  N <- 10
  point_coords <- cbind(
    rep(seq(0, 1, length.out = N), times = N),
    rep(seq(0, 1, length.out = N), each  = N)
  )

  points <- sf::st_multipoint(point_coords)
  plot(points)


  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # An outline of a polygon (making a rect to keep thing simple)
  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  xs <- c(0.1, 0.9, 0.9, 0.1, 0.1)
  ys <- c(0.1, 0.1, 0.9, 0.9, 0.1)
  poly <- sf::st_linestring(cbind(xs, ys))
  plot(poly)


  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # simple plot of the polygon outline and the points
  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  coll <- st_geometrycollection(list(points, poly))
  plot(coll)


  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # Which points are close to the outline?
  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  points_sfc <- sf::st_cast(st_sfc(points), 'POINT')
  close_indices    <- sf::st_is_within_distance(points_sfc, poly, dist = 0.1)
  close_bool       <- lengths(close_indices) > 0
  close_points_sfc <- points_sfc[close_bool]
  close_coords     <- sf::st_coordinates(close_points_sfc)


  close_points <- sf::st_multipoint(close_coords)
  plot(close_points)


  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # simple plot of the polygon outline and the points
  #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  coll <- st_geometrycollection(list(close_points, poly))
  plot(coll)

Hope it makes sense:

# packages
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, PROJ 4.9.3

# define some points
N <- 10
point_coords <- cbind(
  rep(seq(0, 1, length.out = N), times = N),
  rep(seq(0, 1, length.out = N), each  = N)
)
multi_points <- st_sfc(st_multipoint(point_coords))
points <- st_cast(multi_points, "POINT")

# define a LINESTRING boundary
xs <- c(0.1, 0.9, 0.9, 0.1, 0.1)
ys <- c(0.1, 0.1, 0.9, 0.9, 0.1)
poly <- st_linestring(cbind(xs, ys))

# filter only points within a buffer of 0.1
in_points <- points[st_intersects(points, st_buffer(poly, 0.1), sparse = FALSE)]
plot(in_points)
plot(poly, add = TRUE, col = "red")

# define a POLYGONAL boundary
poly <- st_polygon(list(cbind(xs, ys)))
plot(poly, col = "red")

# extract the boundary
poly_boundary <- st_boundary(poly)

# repeat the same procedure
in_points <- points[st_intersects(points, st_buffer(poly_boundary, 0.1), sparse = FALSE)]
plot(in_points)
plot(poly_boundary, add = TRUE, col = "red")

^{Created on 2020-02-18 by the reprex package (v0.3.0)}