Places rectangular quadrats by first generating a dense set of random seed points inside the domain, computing the Voronoi tessellation, and then identifying those Voronoi cells whose largest inscribed circle is big enough to contain the requested quadrat (by diagonal). For each suitable cell, the quadrat is placed at the cell’s “pole of inaccessibility” (center of the inscribed circle), which tends to yield well‑spaced sampling locations even in irregular polygons.
Arguments
- domain
An sf polygon/multipolygon defining the sampling region. Must carry a valid projected CRS suitable for linear measurements.
- n_quadrats
Integer (≥1). Target number of quadrats to place. The actual number returned may be smaller if the domain cannot accommodate enough suitable Voronoi cells.
- quadrat_size
Numeric vector of length 2, c(width, height), giving quadrat dimensions in the same linear units as domain (e.g., meters). Both values must be positive and finite.
- voronoi_seed_factor
Numeric (≥1 recommended). Multiplier controlling how many random seeds are used to build the Voronoi diagram: n_seeds = n_quadrats * voronoi_seed_factor. Use larger values (e.g., 8–20) in very irregular or narrow domains to improve coverage.
Value
An sf object (polygons) with:
- quadrat_id
Sequential integer identifier of the placed quadrats.
- geometry
Axis‑aligned rectangular polygon for each quadrat.
Details
The function assumes a projected (planar) CRS for domain so that distances/areas are in linear units. If your data are in longitude/latitude, reproject (e.g., to UTM) before calling.
The number of initial seeds is n_quadrats * voronoi_seed_factor. Larger values explore the domain more densely and typically find more suitable cells, at the cost of extra computation.
Suitability is decided by comparing each cell’s inscribed‑circle radius to half the requested quadrat diagonal: radius >= sqrt(width^2 + height^2) / 2.
If fewer than n_quadrats suitable cells exist, all suitable cells are used and a warning is issued.
Quadrats are axis‑aligned rectangles centered at the selected cell centers. Because the cell centers can be near each other, axis‑aligned rectangles may still overlap slightly in tight spaces—even when circles do not. If strict non‑overlap is required, post‑filtering is recommended.
Internally uses sf::st_voronoi() for tessellation and st_inscribed_circle() (provided by lwgeom) to compute maximal in‑circle centers/radii.
See also
place_quadrats (random, non‑overlapping),
place_quadrats_tiled (systematic tiling),
place_quadrats_systematic (grid centers within domain),
place_quadrats_transect (parallel transects)