stratified.random {spatialEco}

Stratified random sample

Description

Creates a stratified random sample of an sp class object

Usage

stratified.random(x, strata, n = 10, reps = 1, replace = FALSE)

Arguments

x

An sf class object

strata

Column in x with stratification factor

n

Number of random samples

reps

Number of replicates per strata

replace

(TRUE/FALSE) Sampling with replacement

Details

If replace=FALSE features are removed from consideration in subsequent replicates. Conversely, if replace=TRUE, a feature can be selected multiple times across replicates. Not applicable if rep=1.

Value

An sf class object containing random samples

Author(s)

Jeffrey S. Evans <[e-mail]>

References

Hudak, A.T., N.L. Crookston, J.S. Evans, M.J. Falkowski, A.M.S. Smith, P. Gessler and P. Morgan. (2006) Regression modelling and mapping of coniferous forest basal area and tree density from discrete-return lidar and multispectral satellite data. Canadian Journal of Remote Sensing 32: 126-138.

Examples

if(require(sp, quietly = TRUE)) {
library(sf)
  data(meuse, package = "sp")
  meuse <- st_as_sf(meuse, coords = c("x", "y"), crs = 28992, 
                    agr = "constant")

# Create stratified variable using quartile breaks
x1 <- cut(meuse$cadmium, summary(meuse$cadmium)[-4], 
          include.lowest=TRUE)
  levels(x1) <- seq(1,nlevels(x1),1)
x2 <- cut(meuse$lead, summary(meuse$lead)[-4], 
          include.lowest=TRUE)
  levels(x2) <- seq(1,nlevels(x2),1) 
meuse$STRAT <- paste(x1, x2, sep='.')

# Counts for each full strata (note; 2 strata have only 1 observation)
tapply(meuse$STRAT, meuse$STRAT, length)
   
# 2 replicates, 2 samples with replacement
ssample <- stratified.random(meuse, strata='STRAT', n=2, reps=2, 
                             replace=TRUE)
  tapply(ssample$STRAT, ssample$STRAT, length)

# 2 replicates, 2 samples no replacement
ssample.nr <- stratified.random(meuse, strata='STRAT', n=2, reps=2)
  tapply(ssample.nr$STRAT, ssample.nr$STRAT, length)

# n=1 and reps=10 for sequential numbering of samples 
ssample.ct <- stratified.random(meuse, strata='STRAT', n=1, reps=10, 
                                replace=TRUE)
  tapply(ssample.ct$STRAT, ssample.ct$STRAT, length)

# Plot random samples colored by replacement
ssample$REP <- factor(ssample$REP)
  plot(ssample['REP'], pch=20)

} else { 
  cat("Please install sp package to run example", "\n")
}