bas.point: Draws a Balanced Acceptance Sample (BAS) from a discrete...

In SDraw: Spatially Balanced Samples of Spatial Objects

Description

Draws a BAS sample from a SpatialPoints* object.

Usage

bas.point(x, n)

Arguments

x	A SpatialPoints or SpatialPointsDataFrame object. This object must contain at least 1 point.
n	Sample size. Number of points to select from the set of points contained in x.

Details

The BAS method for points computes the minimum distance between any two points in x and places a small square (pixel) around each. Size of the square around each point is d/sqrt(2) on a side, where d is the minimum distance between points. The BAS method for points then selects a BAS sample from the set of polygons (i.e., squares) surrounding each point (see bas.polygon). The BAS method of polygons selects Halton points until n points are located inside the squares surrounding the points. When a square contains a Halton point, the official sample location is the the original point (center of the square), not the Halton point.

Value

A SpatialPointsDataFrame containing locations 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 point in x that has been selected. The ID of points in x are row.names(x).

• Any attributes of the original lines (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., "point").

• 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 interest. 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).

Author(s)

Trent McDonald

See Also

bas.polygon, bas.line, spsample

Examples

## Not run: 
bas.point( WA.cities, 100)

## End(Not run)

SDraw documentation built on July 8, 2020, 6:23 p.m.