poly2nb

poly2nb is a function in the spdep package for R that constructs a neighbor list (an object of class nb) representing contiguity-based adjacency among polygonal features. It takes a SpatialPolygons or SpatialPolygonsDataFrame object (from the sp package) and returns, for each polygon, the indices of neighboring polygons.

Adjacency is defined by contiguity between polygons. By default, poly2nb uses rook contiguity, where two polygons are considered neighbors if they share a boundary of non-zero length. A common option is to use queen contiguity (setting queen = TRUE), which also treats polygons that touch at a single point (vertex) as neighbors. The resulting nb object can be converted to a spatial weights matrix for use in spatial econometric models and analyses, for example with nb2listw or nb2mat.

The function accepts optional arguments to control labeling and handling of geometries, and it relies on topological tests to determine adjacency. It is applicable to polygons with holes or to disjoint sets of polygons, though polygons that do not touch any others will have empty neighbor lists. Users should ensure the input polygons are valid topologically to avoid errors.

Typical uses include creating neighborhood structures for regional analysis, spatial autocorrelation studies, and various spatial regression models (such as SAR or CAR models). poly2nb is a foundational step for translating polygon geometries into neighbor relationships that drive many spatial analyses, and it serves as a bridge between polygon data and weighted spatial representations.