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Sampling based on a stratified raster.


  allocation = "prop",
  method = "Queinnec",
  weights = NULL,
  force = FALSE,
  mraster = NULL,
  mindist = NULL,
  existing = NULL,
  include = FALSE,
  remove = FALSE,
  access = NULL,
  buff_inner = NULL,
  buff_outer = NULL,
  wrow = 3,
  wcol = 3,
  plot = FALSE,
  details = FALSE,
  filename = NULL,
  overwrite = FALSE



spatRaster. Stratification raster to be used for sampling.


Numeric. Number of desired samples. existing, include and force influence this value.


Character. Allocation algorithm to be used. Either prop (default) for proportional allocation, optim for optimal allocation (equal sampling cost), equal for equal number of samples (defined by nSamp) for each strata, or "manual" for user defined strata weights defined using weights.


Character. Sampling design approach "Queinnec" (default) or "random". "Queinnec" method is described in notes below. "random" performs traditional stratified random sampling where probability to sample each cell within each stratum is equal assuming default parameters for mindist. existing, include, remove are ignored when method = "random".


Numeric. Only applicable when allocation = "manual". Vector of weights where sum(weights) == 1. Vector length must be equal to the number of unique strata where the first numeric value corresponds to stratum 1, second stratum 2 etc.


Logical. Default = FALSE - force nSamp to be exactly the user defined value in cases where nSamp and sraster strata count are not equally divisible. Additional samples often need to be allocated or removed based on rounding differences resulting from proportional differences between nSamp and strata coverages in sraster. In these instances samples are either added to strata with the lowest number of samples or are removed from strata with the highest number of samples. Has no effect when existing is provided.


spatRaster. ALS metric raster. Required when allocation = optim.


Numeric. Minimum allowable distance between selected samples. Default = NULL.


sf 'POINT' or data.frame. Existing plot network.


Logical. If TRUE include existing plots in nSamp total.


Logical. If TRUE randomly remove samples from over represented strata to meet allocated sample numbers. Used only when existing and include are both TRUE.


sf 'LINESTRING' or 'MULTILINESTRING'. Access network.


Numeric. Inner buffer boundary specifying distance from access where plots cannot be sampled.


Numeric. Outer buffer boundary specifying distance from access where plots can be sampled.


Numeric. Number of row in the focal window (default = 3).


Numeric. Number of columns in the focal window (default = 3).


Logical. Plots samples of type `existing` (if provided; croses) and `new` (circles) along with sraster.


Logical. If FALSE (default) output is sf object of stratified samples. If TRUE return a list where $details additional sampling information and $raster is an sf object of stratified samples.


Character. Path to write output samples.


Logical. Choice to overwrite existing filename if it exists.


An sf object with nSamp stratified samples.


The sampling is performed in 2 stages when method = "Queinnec":

  1. Rule 1 - Sample within grouped stratum pixels defined within the wrow, wcol parameters

  2. Rule 2 - If no samples exist to satisfy Rule 1 individual stratum pixels are sampled.

    The rule applied to allocate each sample is defined in the rule attribute of output samples.

existing may contain samples that fall in sraster cells that are `NA`. If this occurs and include = TRUE, `NA` samples are separated during sampling and re-appended at the end of the sampling process.

If the sraster provided contains factor values, the algorithm will automatically convert these into the numeric factor levels and perform sampling using those values. The categories (factor values) will be extracted and appended to the algorithm output as the `category` attribute.


Queinnec, M., White, J. C., & Coops, N. C. (2021). Comparing airborne and spaceborne photon-counting LiDAR canopy structural estimates across different boreal forest types. Remote Sensing of Environment, 262 (August 2020), 112510.


Tristan R.H. Goodbody & Martin Queinnec


#--- Load raster and access files ---#
r <- system.file("extdata", "sraster.tif", package = "sgsR")
sr <- terra::rast(r)

#--- perform stratified sampling random sampling ---#
sraster <- sample_strat(
  sraster = sr,
  nSamp = 50
#> Using 'Queinnec' sampling method.
#> Implementing proportional allocation of samples.
#> nSamp of 50 is not perfectly divisible based on strata distribution. nSamp of 51 will be returned. Use 'force = TRUE' to brute force to 50.
#> Processing strata : 1
#> Processing strata : 2
#> Processing strata : 3
#> Processing strata : 4