background: Background sample

Description

Creates a point sample that can be used as a NULL for SDM's and other modeling approaches.

Usage

background(
  x,
  p = 1000,
  known = NULL,
  d = NULL,
  type = c("regular", "random", "hexagon", "nonaligned")
)

Value

A sf POINT feature class or data.frame with x,y coordinates

Arguments

x: A sf class polygon defining sample region
p: Size of sample
known: An sf POINT class of known locations with same CSR as x
d: Threshold distance for known proximity
type: Type of sample c("systematic", "random", "hexagon", "nonaligned")

Author

Jeffrey S. Evans <jeffrey_evans@tnc.org>

Details

This function creates a background point sample based on an extent or polygon sampling region. The known argument can be used with d to remove sample points based on distance-based proximity to existing locations (eg., known species locations). The size (p) of the resulting sample will be dependent on the known locations and the influence of the distance threshold (d). As such, if the know and d arguments are provided the exact value provided in p will not be returned.

Examples

Run this code

library(sf)

# define study area
sa <- suppressWarnings(st_cast(st_read(
        system.file("shape/nc.shp", 
        package="sf")), "POLYGON"))
  sa <- sa[10,]

# create "known" locations  
locs <- st_sample(sa, 50)
  st_crs(locs) <- st_crs(sa)

# systematic sample using extent polygon
e <- st_as_sf(st_as_sfc(st_bbox(sa)))
  st_crs(e) <- st_crs(sa)
s <- background(e, p=1000, known=locs, d=1000)
  plot(st_geometry(s), pch=20)
    plot(st_geometry(locs), pch=20, col="red", add=TRUE)

# systematic sample using irregular polygon
s <- background(sa, p=1000, known=locs, d=1000)
  plot(st_geometry(sa)) 
    plot(st_geometry(s), pch=20, add=TRUE)
      plot(st_geometry(locs), pch=20, col="red", add=TRUE)

# random sample using irregular polygon
s <- background(sa, p=500, known=locs, 
                d=1000, type="random")
  plot(st_geometry(sa)) 
    plot(st_geometry(s), pch=20, add=TRUE)
      plot(st_geometry(locs), pch=20, col="red", add=TRUE)

Run the code above in your browser using DataLab