bas.polygon: Draws a Balanced Acceptance Sample (BAS) from an area...
In SDraw: Spatially Balanced Samples of Spatial Objects

Description Usage Arguments Details Value Author(s) References See Also Examples

Draws a BAS sample from a SpatialPolygons* object

1	bas.polygon(x, n)

`x`	A `SpatialPolygons` or `SpatialPolygonsDataFrame` object. This object must contain at least 1 polygon. If it contains more than 1 polygon, the BAS sample is drawn from the union of all.
`n`	Sample size. Number of locations to draw from the union of all polygons contained in `x`.

A BAS sample is drawn from the union of all polygons in x by enclosing all polygons in a bounding square and selecting a randomized Halton sequence of points from the bounding square. Points falling outside all polygons are discarded until exactly n locations are selected inside the polygons.

The sampling frame for routine is infinite and contains all (infinitesimally small) points in the union of polygons in x.

A SDrawSample object, which is a SpatialPointsDataFrame object containing points in the BAS sample, in BAS order. Attributes of the sample points are:

sampleID: A unique identifier for every sample point. This encodes the BAS order. If BAS order is lost, return[ order( return$sampleID ),] will resort the returned object (i.e., return) into BAS order.
geometryID: The ID of the polygon in x which each sample point falls. The ID of polygons in x are row.names(geometry(x)).
Any attributes of the original polygons (in x).

Additional attributes of the output object, beyond those which make it a SpatialPointsDataFrame, are:

frame: Name of the input sampling frame.
frame.type: Type of resource in sampling frame. (i.e., "polygon").
sample.type: Type of sample drawn. (i.e., "BAS").
random.start: The random seed of the random-start Halton sequence that produced the sample. This is a vector of length 2 whose elements are random integers between 0 and maxU. This routine ensures that the point associated with this index falls inside a polygon of x. i.e., that halton(1,2,random.start) scaled by a square bounding box (see attribute bas.bbox below) lies inside a polygon of x.

Note that halton(1,2,random.start+i), for i > 0, is not guaranteed to fall inside a polygon of x when scaled by bas.bbox. The sample consists of the point associated with random.start and the next n-1 Halton points in sequence that fall inside a polygon of x.
bas.bbox: The square bounding box surrounding x used to scale Halton points. A scaled Halton sequence of n points is bas.bbox[,"min"] + t(halton(n,2,random.start)) * rep( max(diff(t(bas.bbox))), 2).

Trent McDonald

Robertson, B.L., J. A. Brown, T. L. McDonald, and P. Jaksons (2013) "BAS: Balanced Acceptance Sampling of Natural Resources", Biometrics, v69, p. 776-784.

bas.line, bas.point, sdraw

#   Draw sample
WA_sample <- bas.polygon(WA, 100)  

#   Plot
plot( WA )

# Plot first 100 sample locations
points( WA_sample[ WA_sample$siteID <= 100, ], pch=16 ) 

# Plot second 100 locations 
points( WA_sample[ WA_sample$siteID >  100, ], pch=1 )