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Systematic stratified sampling

Source: R/sample_sys_strat.R
sample_sys_strat.Rd

Systematic stratified sampling with random start point and translation within a square or hexagonal tessellation for each stratum.

Usage

sample_sys_strat(
  sraster,
  cellsize,
  square = TRUE,
  location = "centers",
  force = FALSE,
  access = NULL,
  buff_inner = NULL,
  buff_outer = NULL,
  plot = FALSE,
  filename = NULL,
  overwrite = FALSE,
  details = FALSE
)

Arguments

sraster

spatRaster. Stratified raster with name "strata".

cellsize

Numeric. Desired cellsize for tessellation.

square

Logical. Tessellation shape. Default is regular square grid, if FALSE hexagons are used.

location

Character. Sample location within tessellation. Default = "centers") returns samples at tessellation centers, "corners" - corners of tessellation are returned, "random" - samples are randomly located within tessellations.

force

Logical. Only applies when location = "random". If TRUE, random samples are forced to fall in areas where raster does not have NA values. This will considerably slow processing.

access

sf. Road access network - must be lines.

buff_inner

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

buff_outer

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

plot

Logical. Plots output strata raster with samples.

filename

Character. Path to write output samples.

overwrite

Logical. Choice to overwrite existing filename if it exists.

details

Logical. If FALSE (default) output is sf object of systematic samples. If TRUE returns a list of sf objects where tessellation is the tessellation grid for sampling, and samples are the systematic samples.

Value

An sf object with sampled points over unique tessellations.

Note

Specifying location = "random" can result in tessellations with no samples. This results from raster have NA values at the random location chosen. Using force = TRUE removes areas of NA from sampling entirely, but considerably slows processing speeds. Thanks to R. Hijmans for help in debugging and providing suggestions for this script.

All stratum are sampled using random tessellation start points and translations.

See also

Other sample functions: sample_ahels(), sample_balanced(), sample_clhs(), sample_existing(), sample_nc(), sample_srs(), sample_strat(), sample_systematic()

Author

Tristan R.H. Goodbody

Examples

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

#--- perform grid sampling ---#
sample_sys_strat(
  sraster = sr,
  cellsize = 1000
)
#> Processing strata : 1
#> Warning: [extract] source already open for reading
#> Processing strata : 2
#> Warning: [extract] source already open for reading
#> Processing strata : 3
#> Warning: [extract] source already open for reading
#> Processing strata : 4
#> Warning: [extract] source already open for reading
#> Simple feature collection with 39 features and 1 field
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 431113.6 ymin: 5337828 xmax: 438487.1 ymax: 5343203
#> Projected CRS: UTM Zone 17, Northern Hemisphere
#> First 10 features:
#>    strata                 geometry
#> 1       1   POINT (436164 5337828)
#> 2       1 POINT (437141.6 5338038)
#> 3       1 POINT (437908.8 5339226)
#> 4       1 POINT (433367.2 5341317)
#> 5       1 POINT (434344.8 5341528)
#> 6       1   POINT (435112 5342716)
#> 7       1 POINT (436089.6 5342926)
#> 8       2 POINT (432529.9 5342241)
#> 9       2 POINT (432558.4 5341242)
#> 10      2 POINT (433529.5 5342270)

sample_sys_strat(
  sraster = sr,
  cellsize = 1000,
  square = FALSE,
  location = "corners"
)
#> Processing strata : 1
#> Warning: [extract] source already open for reading
#> Processing strata : 2
#> Warning: [extract] source already open for reading
#> Processing strata : 3
#> Warning: [extract] source already open for reading
#> Processing strata : 4
#> Warning: [extract] source already open for reading
#> Simple feature collection with 223 features and 1 field
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 431127.7 ymin: 5337718 xmax: 438206.3 ymax: 5343172
#> Projected CRS: UTM Zone 17, Northern Hemisphere
#> First 10 features:
#>    strata                 geometry
#> 1       1 POINT (438177.9 5338179)
#> 2       1 POINT (438177.9 5338179)
#> 3       1 POINT (437864.4 5338663)
#> 4       1 POINT (438127.5 5339177)
#> 5       1 POINT (438127.5 5339177)
#> 6       1 POINT (438077.1 5340176)
#> 7       1 POINT (437864.4 5338663)
#> 8       1 POINT (438177.9 5338179)
#> 9       1 POINT (437024.6 5338120)
#> 10      1 POINT (437864.4 5338663)