# spsample

##### sample point locations in (or on) a spatial object

sample point locations within a square area, a grid, a polygon, or on a spatial line, using regular or random sampling methods; the methods used assume that the geometry used is not spherical, so objects should be in planar coordinates

##### Usage

```
spsample(x, n, type, ...)
makegrid(x, n = 10000, nsig = 2, cellsize, offset = rep(0.5, nrow(bb)),
pretty = TRUE)
```

##### Arguments

- x
Spatial object;

`spsample(x,...)`

is a generic method for the existing`sample.Xxx`

functions- ...
optional arguments, passed to the appropriate

`sample.Xxx`

functions; see NOTES for`nclusters`

and`iter`

- n
(approximate) sample size

- type
character;

`"random"`

for completely spatial random;`"regular"`

for regular (systematically aligned) sampling;`"stratified"`

for stratified random (one single random location in each "cell");`"nonaligned"`

for nonaligned systematic sampling (nx random y coordinates, ny random x coordinates);`"hexagonal"`

for sampling on a hexagonal lattice;`"clustered"`

for clustered sampling;`"Fibonacci"`

for Fibonacci sampling on the sphere (see references).- bb
bounding box of the sampled domain; setting this to a smaller value leads to sub-region sampling

- offset
for square cell-based sampling types (regular, stratified, nonaligned, hexagonal): the offset (position) of the regular grid; the default for

`spsample`

methods is a random location in the unit cell [0,1] x [0,1], leading to a different grid after each call; if this is set to`c(0.5,0.5)`

, the returned grid is not random (but, in Ripley's wording, "centric systematic"). For line objects, a single offset value is taken, where the value varies within the [0, 1] interval, and 0 is the beginning of each Line object, and 1 its end- cellsize
if missing, a cell size is derived from the sample size

`n`

; otherwise, this cell size is used for all sampling methods except`"random"`

- nsig
for "pretty" cell size;

`spsample`

does not result in pretty grids- pretty
logical; if

`TRUE`

, choose pretty (rounded) coordinates

##### Value

an object of class SpatialPoints-class. The number of
points is only guaranteed to equal `n`

when sampling is done in a
square box, i.e. (`sample.Spatial`

). Otherwise, the obtained number
of points will have expected value `n`

.

When `x`

is of a class deriving from Spatial-class for which
no spsample-methods exists, sampling is done in the bounding box
of the object, using `spsample.Spatial`

. An overlay using
over may be necessary to select the features inside the geometry
afterwards.

Sampling type `"nonaligned"`

is not implemented for line objects.

Some methods may return NULL if no points could be successfully placed.

`makegrid`

makes a regular grid that covers `x`

; when
`cellsize`

is not given it derives one from the number of grid
points requested (approximating the number of cells). It tries to
choose pretty cell size and grid coordinates.

##### Note

If an Polygon-class object has zero area (i.e. is a line), samples on this line element are returned. If the area is very close to zero, the algorithm taken here (generating points in a square area, selecting those inside the polygon) may be very resource intensive. When numbers of points per polygon are small and type="random", the number searched for is inflated to ensure hits, and the points returned sampled among these.

The following two arguments can be further specified:

`nclusters`

Number of clusters (strata) to sample from.

`iter`

(default = 4) number of times to try to place sample points
in a polygon before giving up and returning NULL - this may occur when
trying to hit a small and awkwardly shaped polygon in a large bounding
box with a small number of points

##### Methods

- x = "Spatial"
sample in the bbox of

`x`

- x = "Line"
sample on a line

- x = "Polygon"
sample in a Polygon

- x = "Polygons"
sample in a Polygons object, consisting of possibly multiple Polygon objects (holes must be correctly defined, use

`checkPolygonsHoles`

if need be)- x = "SpatialPolygons"
sample in an SpatialPolygons object; sampling takes place over all Polygons objects present, use subsetting to vary sampling intensity (density); holes must be correctly defined, use

`checkPolygonsHoles`

if need be- x = "SpatialGrid"
sample in an SpatialGrid object

- x = "SpatialPixels"
sample in an SpatialPixels object

##### References

Chapter 3 in B.D. Ripley, 1981. Spatial Statistics, Wiley

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

##### See Also

##### Examples

```
# NOT RUN {
data(meuse.riv)
meuse.sr = SpatialPolygons(list(Polygons(list(Polygon(meuse.riv)), "x")))
plot(meuse.sr)
points(spsample(meuse.sr, n = 1000, "regular"), pch = 3)
plot(meuse.sr)
points(spsample(meuse.sr, n = 1000, "random"), pch = 3)
plot(meuse.sr)
points(spsample(meuse.sr, n = 1000, "stratified"), pch = 3)
plot(meuse.sr)
points(spsample(meuse.sr, n = 1000, "nonaligned"), pch = 3)
plot(meuse.sr)
points(spsample(meuse.sr@polygons[[1]], n = 100, "stratified"), pch = 3, cex=.5)
data(meuse.grid)
gridded(meuse.grid) = ~x+y
image(meuse.grid)
points(spsample(meuse.grid,n=1000,type="random"), pch=3, cex=.5)
image(meuse.grid)
points(spsample(meuse.grid,n=1000,type="stratified"), pch=3, cex=.5)
image(meuse.grid)
points(spsample(meuse.grid,n=1000,type="regular"), pch=3, cex=.5)
image(meuse.grid)
points(spsample(meuse.grid,n=1000,type="nonaligned"), pch=3, cex=.5)
fullgrid(meuse.grid) = TRUE
image(meuse.grid)
points(spsample(meuse.grid,n=1000,type="stratified"), pch=3,cex=.5)
# }
```

*Documentation reproduced from package sp, version 1.2-7, License: GPL (>= 2)*