Sampling

Let’s start by attaching the necessary packages and setting the seed.

library(simsam)
library(terra)
#> terra 1.8.80
set.seed(2024-11-20)

Next, we define the raster grid to sample points/locations from.

rast_grid = terra::rast(xmin = 0, xmax = 200, ymin = 0, ymax = 200,
                        ncols = 200, nrows = 200)

Alternatively, we can read a raster from a file.

Sample

The simsam package supports several types of sampling, including jittered, clustered, and random with the sam_field() function. All these types of sampling require the rast_grid object, the number of samples to create (size), and the type of sampling (type). Type-specific options are passed via the type_opts list parameter.

The jittered sampling is a regular sample moved by an amount of noise. The amount option in type_opts controls the amount of jitter. Importantly, this sampling type ensures that all the samples fall within the sampling window.

sample_j1 = sam_field(rast_grid, size = 100, type = "jittered", type_opts = list(amount = 5))
plot(sample_j1)

sample_j2 = sam_field(rast_grid, size = 100, type = "jittered", type_opts = list(amount = 20))
plot(sample_j2)

The clustered sampling creates samples forming groups of points with similar locations. The outcome is controlled by the nclusters and radius options in type_opts, where the former controls the number of clusters and the latter the radius of the buffer around each cluster.

sample_c1 = sam_field(rast_grid, 100, "clustered", type_opts = list(nclusters = 5, radius = 5))
plot(sample_c1)

sample_c2 = sam_field(rast_grid, 100, "clustered", type_opts = list(nclusters = 5, radius = 10))
plot(sample_c2)

Random sampling creates samples with no spatial structure.

sample_r = sam_field(rast_grid, 100, "random")
plot(sample_r)

Moreover, the sam_field() function can be used with additional arguments passed to the terra::spatSample() function, when the type is not specified. In such cases, the method argument can be used to control the sampling method, e.g., method = "regular" or method = "random" and the exhaustive argument to sample only from not NA areas in the input raster.

sample_u1 = sam_field(rast_grid, 100, method = "regular")
plot(sample_u1)

sample_u2 = sam_field(rast_grid, 100, method = "random", exhaustive = TRUE)
plot(sample_u2)

Jakub Nowosad

2025-12-01

Sample