% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/sample.R
\name{st_sample}
\alias{st_sample}
\alias{st_sample.sf}
\alias{st_sample.sfc}
\alias{st_sample.sfg}
\title{sample points on or in (sets of) spatial features}
\usage{
st_sample(x, size, ...)

\method{st_sample}{sf}(x, size, ...)

\method{st_sample}{sfc}(
  x,
  size,
  ...,
  type = "random",
  exact = TRUE,
  warn_if_not_integer = TRUE,
  by_polygon = FALSE,
  progress = FALSE
)

\method{st_sample}{sfg}(x, size, ...)
}
\arguments{
\item{x}{object of class \code{sf} or \code{sfc}}

\item{size}{sample size(s) requested; either total size, or a numeric vector with sample sizes for each feature geometry. When sampling polygons, the returned sampling size may differ from the requested size, as the bounding box is sampled, and sampled points intersecting the polygon are returned.}

\item{...}{passed on to \link[base]{sample} for \code{multipoint} sampling, or to \code{spatstat} functions for spatstat sampling types (see details)}

\item{type}{character; indicates the spatial sampling type; one of \code{random}, \code{hexagonal} (triangular really), \code{regular}, \code{Fibonacci},
or one of the \code{spatstat} methods such as \code{Thomas} for calling \code{spatstat.random::rThomas} (see Details).}

\item{exact}{logical; should the length of output be exactly}

\item{warn_if_not_integer}{logical; if \code{FALSE} then no warning is emitted if \code{size} is not an integer}

\item{by_polygon}{logical; for \code{MULTIPOLYGON} geometries, should the effort be split by \code{POLYGON}? See https://github.com/r-spatial/sf/issues/1480
the same as specified by \code{size}? \code{TRUE} by default. Only applies to polygons, and
when \code{type = "random"}.}

\item{progress}{logical; if \code{TRUE} show progress bar (only if \code{size} is a vector).}
}
\value{
an \code{sfc} object containing the sampled \code{POINT} geometries
}
\description{
Sample points on or in (sets of) spatial features.
By default, returns a pre-specified number of points that is equal to
\code{size} (if \code{type = "random"} and \code{exact = TRUE}) or an approximation of
\code{size} otherwise. \code{spatstat} methods are
interfaced and do not use the \code{size} argument, see examples.
}
\details{
The function is vectorised: it samples \code{size} points across all geometries in
the object if \code{size} is a single number, or the specified number of points
in each feature if \code{size} is a vector of integers equal in length to the geometry
of \code{x}.

if \code{x} has dimension 2 (polygons) and geographical coordinates (long/lat), uniform random sampling on the sphere is applied, see e.g. \url{http://mathworld.wolfram.com/SpherePointPicking.html}. 

For \code{regular} or \code{hexagonal} sampling of polygons, the resulting size is only an approximation.

As parameter called \code{offset} can be passed to control ("fix") regular or hexagonal sampling: for polygons a length 2 numeric vector (by default: a random point from \code{st_bbox(x)}); for lines use a number like \code{runif(1)}.

Fibonacci sampling see: Alvaro Gonzalez, 2010. Measurement of Areas on a Sphere Using Fibonacci and Latitude-Longitude Lattices. 
Mathematical Geosciences 42(1), p. 49-64

Sampling methods from package \code{spatstat} are interfaced (see examples), and need their own parameters to be set. 
For instance, to use \code{spatstat.random::rThomas()}, set \code{type = "Thomas"}.
}
\examples{
nc = st_read(system.file("shape/nc.shp", package="sf"))
p1 = st_sample(nc[1:3, ], 6)
p2 = st_sample(nc[1:3, ], 1:3)
plot(st_geometry(nc)[1:3])
plot(p1, add = TRUE)
plot(p2, add = TRUE, pch = 2)
x = st_sfc(st_polygon(list(rbind(c(0,0),c(90,0),c(90,90),c(0,90),c(0,0)))), crs = st_crs(4326))
plot(x, axes = TRUE, graticule = TRUE)
if (sf_extSoftVersion()["proj.4"] >= "4.9.0")
  plot(p <- st_sample(x, 1000), add = TRUE)
if (require(lwgeom, quietly = TRUE)) { # for st_segmentize()
x2 = st_transform(st_segmentize(x, 1e4), st_crs("+proj=ortho +lat_0=30 +lon_0=45"))
g = st_transform(st_graticule(), st_crs("+proj=ortho +lat_0=30 +lon_0=45"))
plot(x2, graticule = g)
if (sf_extSoftVersion()["proj.4"] >= "4.9.0") {
  p2 = st_transform(p, st_crs("+proj=ortho +lat_0=30 +lon_0=45"))
  plot(p2, add = TRUE)
}
}
x = st_sfc(st_polygon(list(rbind(c(0,0),c(90,0),c(90,10),c(0,90),c(0,0))))) # NOT long/lat:
plot(x)
p_exact = st_sample(x, 1000, exact = TRUE)
p_not_exact = st_sample(x, 1000, exact = FALSE)
length(p_exact); length(p_not_exact)
plot(st_sample(x, 1000), add = TRUE)
x = st_sfc(st_polygon(list(rbind(c(-180,-90),c(180,-90),c(180,90),c(-180,90),c(-180,-90)))),
 crs=st_crs(4326))
# FIXME:
#if (sf_extSoftVersion()["proj.4"] >= "4.9.0") {
#  p = st_sample(x, 1000)
#  st_sample(p, 3)
#}
# hexagonal:
sfc = st_sfc(st_polygon(list(rbind(c(0,0), c(1,0), c(1,1), c(0,0)))))
plot(sfc)
h = st_sample(sfc, 100, type = "hexagonal")
h1 = st_sample(sfc, 100, type = "hexagonal")
plot(h, add = TRUE)
plot(h1, col = 'red', add = TRUE)
c(length(h), length(h1)) # approximate!
pt = st_multipoint(matrix(1:20,,2))
ls = st_sfc(st_linestring(rbind(c(0,0),c(0,1))),
 st_linestring(rbind(c(0,0),c(.1,0))),
 st_linestring(rbind(c(0,1),c(.1,1))),
 st_linestring(rbind(c(2,2),c(2,2.00001))))
st_sample(ls, 80)
plot(st_sample(ls, 80))
# spatstat example:
if (require(spatstat.random)) {
 x <- sf::st_sfc(sf::st_polygon(list(rbind(c(0, 0), c(10, 0), c(10, 10), c(0, 0)))))
 # for spatstat.random::rThomas(), set type = "Thomas":
 pts <- st_sample(x, kappa = 1, mu = 10, scale = 0.1, type = "Thomas") 
}
}
