Draw a systematic sample from a SpatialPoints* object or a data.frame.
SpatialPoints* objects can represent point resources in 2-dimensional space, such as towns, event locations,
or grid cell centers.
srs.point(x, n)A SpatialLines, SpatialLinesDataFrame, or data.frame object.
Sample size. Number of points or rows to draw from x.
If n exceeds the number of units (= number of rows in data.frame(x)),
a census is taken (i.e., x is returned).
If input x inherits from a the SpatialPoints class, a
SpatialPointsDataFrame object containing locations and attributes 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., "SRS").
When x is a data frame, the simple random sample is drawn from the rows. That is,
each row is viewed as a sample unit.
This draws equi-probable sample. First order inclusion probabilities are n/N for all units.
# NOT RUN {
# Draw systematic sample across range of population
WA.samp <- srs.point( WA.cities, 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 <- srs.point( df, 5 )
# }
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