calc_elongation: Calculate Elongation Ratio from Minimum Bounding Rectangle

In geomattR: Calculate Geometric Attributes of Spatial Polygons

Calculate Elongation Ratio from Minimum Bounding Rectangle

Description

Calculates the elongation ratio of a polygon based on its minimum bounding rectangle. The minimum bounding rectangle is computed from the convex hull of the input polygon.

Usage

calc_elongation(
  v,
  isHull = FALSE,
  output = "value",
  method = "geo",
  by_feature = FALSE
)

Arguments

v	A SpatVector object representing a polygon, or a pre-computed convex hull if isHull = TRUE.
isHull	Logical. If TRUE, v is treated as a pre-computed convex hull. If FALSE (default), the convex hull is computed internally.
output	Character string. Either "value" (default) to return the elongation ratio, or "polygon" to return the input geometry with elongation_rectangle added as an attribute.
method	Character string passed to terra::distance(). Defaults to "geo" (recommended). Other supported options are "haversine" and "cosine". All three are lon/lat great-circle methods; on a projected CRS, method is ignored and distances are Cartesian. See distance for more information.
by_feature	Logical. If FALSE (default), compute a single response from the convex hull of the whole input set. If TRUE, compute one response per polygon feature.

Details

In the current implementation, elongation is computed as the ratio of the mean of the two largest side lengths to the mean of the two shortest side lengths: E = \frac{\text{mean}(\text{long sides})}{\text{mean}(\text{short sides})}

The minimum bounding rectangle is a planar (GEOS) construction. For lon/lat input it is computed in a local Lambert azimuthal equal-area projection centred on the feature, so that the minimum rectangle is not distorted by the varying length of a degree of longitude; the rectangle is then measured back in the input's CRS with the selected method.

This approximation assumes the minimum bounding rectangle is unique, which may not always be the case. Consider this with caution.

Value

If output = "value", a numeric value representing the elongation ratio of the input polygon from the current minimum-bounding-rectangle approximation. If output = "polygon", the input geometry with elongation_rectangle added. When by_feature = TRUE, value output is returned as a numeric vector (one value per feature). Higher values indicate more elongated shapes.

References

Dražić, Slobodan, Nebojša Ralević, and Joviša Žunić. Shape Elongation from Optimal Encasing Rectangles. Computers & Mathematics with Applications 60, no. 7 (2010): 2035–42. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1016/j.camwa.2010.07.043")}.

Examples

library(terra)
coords <- cbind(c(0, 0, 1, 1, 0), c(0, 1, 1, 0, 0))
polygon <- vect(coords, type = "polygon", crs = "EPSG:4326")
elongation <- calc_elongation(polygon)

# Using a pre-computed convex hull
hull_geom <- terra::hull(polygon, type = "convex")
elongation_hull <- calc_elongation(hull_geom, isHull = TRUE)

geomattR documentation built on Oct. 18, 2026, 5:08 p.m.