Score how much each point looks like an outlier

Returns the local outlier factor of every point in the array, one score per row.

Usage

ga_outlier_scores(geometry, k_neighbours)

Arguments

geometry: a GeoArrow point array
k_neighbours: how many neighbours define a neighbourhood

Value

a double array of scores, the same length as geometry

Details

A score near 1 means a point sits at the same density as its neighbours. Scores meaningfully above 1 mean it is in a sparser neighbourhood than they are, which is what marks an outlier. There is no universal cutoff, so compare scores within a dataset rather than against a fixed threshold.

k_neighbours sets how many neighbours define the local neighbourhood. Small values react to fine structure and large values smooth it away. Scoring is over the whole array, not within each row, so it is a single value. A row that is not a single point, or is null, comes back null.

References

OutlierDetection

Examples

g <- ga_xy(
  c(0, 0.1, 0.2, 0.3, 9),
  c(0, 0.1, 0.2, 0.3, 9)
)

round(as.vector(ga_outlier_scores(g, k_neighbours = 2)), 2)
#> [1]    1    1    1    1 3785