Description Usage Arguments Details Value Author(s) See Also Examples
Draw a systematic sample from a SpatialPoints*
object or a data.frame
.
SpatialPoints*
objects can represent point resources in 2dimensional space, such as towns, event locations,
or grid cell centers.
1  sss.point(x, n)

x 
A 
n 
Sample size. Number of points to draw from the set of all points
in 
The points in x
are systematically sampled in the order
they appear. That is, the sampling frame (i.e., data.frame(x)
) is
not reordered prior to sampling. Each row in the frame represents
a point or sample unit, and rows are sampled systematically starting with row 1.
To draw a systematic sample across the range of an attribute, say attribute y,
sort x
by y prior to calling this routine (e.g,. sss.point( x[order(x$y),], n )
).
This routine draws fixed size systematic samples. Many systematic sampling procedure produce variable size samples. Conceptually, the sample procedure is:
Each sample unit (= row of sample frame) is associated with a line segment. Assuming there
are N units in the frame (N = nrow(x)
), each line segment has length
n/N, where n is the input desired sample size.
Line segments are placed endtoend, starting at 0, in the order in which their associated unit appears in the frame.
To start the systematic sample, the routine choses a random number between 0 and 1. Let this random number be m.
The sample units associated with the line segments containing the numbers m + i for i = 0,1,...,(n1), are selected for the sample.
If input x
inherits from a the SpatialPointsDataFrame
class, a
SpatialPointsDataFrame
object containing locations in the sample is returned.
If input x
is a data.frame
, a data.frame
is returned.
Attributes of the returned sample points are:
sampleID
: A unique identifier for every sample point.
sampleID
starts with 1 at the first point and
increments by one for each.
If x
inherits from SpatialPoints
,
returned points have attribute geometryID
– the ID (=row.names(x)
) of
the sampled point.
Any attributes (columns) associated with the input points (rows).
Additional attributes of the output object are:
frame
: Name of the input sampling frame (i.e., x
).
frame.type
: Type of resource in sampling frame. (i.e., "point").
sample.type
: Type of sample drawn. (i.e., "SSS").
random.start
: The random start for the systematic sample.
Using these additional attributes, one could reconstruct the sample.
Trent McDonald
1 2 3 4 5 6 7 8 9 10 11  # Draw systematic sample across range of population
WA.samp < sss.point( WA.cities[order(WA.cities$POP_2010),], 100 )
plot( WA.cities )
points( WA.samp, col="red", pch=16 )
# Draw systematic sample from data frame
df < data.frame( a=1:100, b=runif(100) )
samp < sss.point( df, 5 )
# Equivalent to simple random sample: randomly sort frame.
samp < sss.point( df[order(df$b),], 5 )

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