Draw a systematic sample from a
SpatialPoints* object or a
SpatialPoints* objects can represent point resources in 2-dimensional space, such as towns, event locations,
or grid cell centers.
Sample size. Number of points to draw from the set of all points
The points in
x are systematically sampled in the order
they appear. That is, the sampling frame (i.e.,
not re-ordered 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,
x by y prior to calling this routine (e.g,.
sss.point( x[order(x$y),], n )).
This routine draws fixed size systematic samples. Many systematic sampling procedure produce variable size samples. Conceptually, the sample procedure is:
Each sample unit (= row of sample frame) is associated with a line segment. Assuming there
are N units in the frame (N =
nrow(x)), each line segment has length
n/N, where n is the input desired sample size.
Line segments are placed end-to-end, starting at 0, in the order in which their associated unit appears in the frame.
To start the systematic sample, the routine choses a random number between 0 and 1. Let this random number be m.
The sample units associated with the line segments containing the numbers m + i for i = 0,1,...,(n-1), are selected for the sample.
x inherits from a the
SpatialPointsDataFrame class, a
SpatialPointsDataFrame object containing locations in the sample is returned.
x is a
data.frame is returned.
Attributes of the returned sample points are:
sampleID: A unique identifier for every sample point.
sampleID starts with 1 at the first point and
increments by one for each.
x inherits from
returned points have attribute
geometryID – the ID (=
the sampled point.
Any attributes (columns) associated with the input points (rows).
Additional attributes of the output object are:
frame: Name of the input sampling frame (i.e.,
frame.type: Type of resource in sampling frame. (i.e., "point").
sample.type: Type of sample drawn. (i.e., "SSS").
random.start: The random start for the systematic sample.
Using these additional attributes, one could reconstruct the sample.
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# Draw systematic sample across range of population WA.samp <- sss.point( WA.cities[order(WA.cities$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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