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
spsample(x, n, type, ...) makegrid(x, n = 10000, nsig = 2, cellsize, offset = rep(0.5, nrow(bb)), pretty = TRUE)
- Spatial object;
spsample(x,...)is a generic method for the existing
- optional arguments, passed to the appropriate
sample.Xxxfunctions; see NOTES for
- (approximate) sample size
"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).
- bounding box of the sampled domain; setting this to a smaller value leads to sub-region sampling
- for square cell-based sampling types (regular, stratified,
nonaligned): the offset (position) of the regular
grid; the default for
spsamplemethods 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
- if missing, a cell size is derived from the sample size
n; otherwise, this cell size is used for all sampling methods except
- for "pretty" cell size;
spsampledoes not result in pretty grids
- logical; if
TRUE, choose pretty (rounded) coordinates
SpatialPoints-class. The number of
points is only guaranteed to equal
nwhen sampling is done in a square box, i.e. (
sample.Spatial). Otherwise, the obtained number of points will have expected value
xis 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.
makegridmakes a regular grid that covers
cellsizeis 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.
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
- x = "Spatial"
- sample in the bbox of
- 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
checkPolygonsHolesif 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
checkPolygonsHolesif need be
- x = "SpatialGrid"
- sample in an SpatialGrid object
- x = "SpatialPixels"
- sample in an SpatialPixels object
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
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[], 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)