Description Usage Arguments Details Value Author(s) See Also Examples
Draws a BAS sample from a SpatialLines*
object.
1 | bas.line(x, n, balance = "1D", init.n.factor = 10)
|
x |
A |
n |
Sample size. Number of locations to draw from the set of all lines
contained in |
balance |
Option specifying how spatial balance is maintained. The options are "1D" or "2D". Under "1D" all lines in Under "2D" a systematic sample of points along the union of all lines
in |
init.n.factor |
If If one desires an underlying grid spaced w meters apart, set
|
If a "1D" sample is requested, spatial balance is maintained on the lines when laid end-to-end in the order they appear. Points far apart in 1 dimension may be close together in 2 dimensions, and vice versa. Thus the sample may not look spatially balanced on a 2D map. This is a true infinite sample in that any of an infinite number of points along the lines could be selected.
If a "2D" BAS sample is requested, spatial balance is maintained
in 2 dimensions. Points are well balance on a 2D map. This is
done by discretization of lines with a dense systematic
sample of points (with
random start) where density of the systematic points is controlled
by init.n.factor
. After
discretization of the line, points are selected
using bas.point
. The BAS method for points places
a small square (pixel) around each and samples the set of squares
using the BAS method for polygons (see bas.polygon
). The BAS
method of polygons computes Halton points until n
fall
inside the squares surrounding discretization points. When a
Halton
point falls in a square, the square is selected and the
sample location is the center of the square (which falls somewhere on the
original lines).
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 line in x
on which each
sample point falls. The
ID of lines 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., "line").
sample.type
: Type of sample drawn. (i.e., "BAS").
balance
: The type of balance ("1d" or "2d").
random.start
: The random seed for the random-start
1D or 2D Halton sequence
that produced the sample.
If balance=="1D"
, this is a single uniform random
integer between 0 and maxU
. If balance=="2D"
, this is
a vector of two uniform random
integers between 0 and maxU
.
bas.bbox
: If balance=="2D"
, this is the
bounding box surrounding x
used to scale Halton points. The scaled Halton sequence is
bas.bbox[,"min"]+
t(halton(n,2,random.start))*
rep(max(
diff(t(bas.bbox))),
2)
.
If balance=="1D"
, this is a vector containing the 1D
bounding box. Lower limit of the 1D bounding box is 0. Upper
limit of the 1D box is the total
length of lines in x
. In this case, Halton points
are scaled as bas.bbox[,"min"]+
halton(n,1,random.start)*
diff(bas.bbox)
which is equivalent to halton(n,1,
random.start)*
bas.bbox[2]
because bas.bbox[1]
is zero.
Trent McDonald
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