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Adaptive AoE Expansion to Capture Minimum Points

Source: R/aoe_expand.R
aoe_expand.Rd

Expands the area of effect just enough to capture at least min_points, subject to hard caps on expansion. This is useful when a fixed scale leaves some supports with insufficient data for stable modelling.

Usage

aoe_expand(
  points,
  support = NULL,
  min_points,
  max_area = 2,
  max_dist = NULL,
  method = c("buffer", "stamp"),
  reference = NULL,
  mask = NULL,
  coords = NULL
)

Arguments

points

An sf object with POINT geometries.

support

One of:

  • sf object with POLYGON/MULTIPOLYGON geometries

  • Country name or ISO code: "France", "FR", "FRA"

  • Vector of countries: c("France", "Germany")

  • Missing: auto-detects countries containing the points

min_points

Minimum number of points to capture in the AoE. The function finds the smallest scale that includes at least this many points.

max_area

Maximum halo area as a proportion of the original support area. Default is 2, meaning halo area cannot exceed twice the support area (total AoE <= 3x original). Set to Inf to disable.

max_dist

Maximum expansion distance in CRS units. For the buffer method, this is the maximum buffer distance. For the stamp method, this is converted to a maximum scale based on the support's characteristic radius. Default is NULL (no distance cap).

method

Method for computing the area of effect:

  • "buffer" (default): Uniform buffer around the support boundary. Robust for any polygon shape. Buffer distance is calculated to achieve the target halo area.

  • "stamp": Scale vertices outward from the centroid (or reference point). Preserves shape proportions but only guarantees containment for star-shaped polygons. May leave small gaps for highly concave shapes.

reference

Optional sf object with a single POINT geometry.

If NULL (default), the centroid of each support is used. Only valid when support has a single row and method = "stamp".

mask

Optional mask for clipping the area of effect. Can be:

  • sf object with POLYGON or MULTIPOLYGON geometry

  • "land": use the bundled global land mask to exclude sea areas If provided, each area of effect is intersected with this mask.

coords

Column names for coordinates when points is a data.frame, e.g. c("lon", "lat"). If NULL, auto-detects common names.

Value

An aoe_result object (same as aoe()) with additional attributes:

target_reached

Logical: was min_points achieved for all supports? Use attr(result, "expansion_info") for per-support details.

expansion_info

Data frame with per-support expansion details: support_id, scale_used, points_captured, target_reached, cap_hit.

Details

Unlike aoe(), which applies consistent geometry across all supports, aoe_expand() adapts the scale per-support based on local point density. Use with caution: this can make AoEs incomparable across regions with different point densities.

Algorithm

For each support, binary search finds the minimum scale where point count >= min_points. The search is bounded by:

  • Lower: scale = 0 (core only)

  • Upper: minimum of max_area cap and max_dist cap

If the caps prevent reaching min_points, a warning is issued and the result uses the maximum allowed scale.

Caps

Two caps ensure AoE doesn't expand unreasonably:

max_area (relative): Limits halo area to max_area times the original. The corresponding scale is sqrt(1 + max_area) - 1. Default max_area = 2 means scale <= 0.732 (total area <= 3x).

max_dist (absolute): Limits expansion distance in CRS units. For buffer method, this is the buffer distance directly. For stamp method, converted to scale via max_dist / characteristic_radius where characteristic_radius = sqrt(area / pi).

See also

aoe() for fixed-scale AoE computation

Examples

library(sf)

# Create a support with sparse points
support <- st_as_sf(
  data.frame(id = 1),
  geometry = st_sfc(st_polygon(list(
    cbind(c(0, 100, 100, 0, 0), c(0, 0, 100, 100, 0))
  ))),
  crs = 32631
)

# Points scattered around
set.seed(42)
pts <- st_as_sf(
  data.frame(id = 1:50),
  geometry = st_sfc(lapply(1:50, function(i) {
    st_point(c(runif(1, -50, 150), runif(1, -50, 150)))
  })),
  crs = 32631
)

# Expand until we have at least 20 points
result <- aoe_expand(pts, support, min_points = 20)

# Check expansion info
attr(result, "expansion_info")