The function aggregates Polygons in a SpatialPolygons object, according to the IDs vector specifying which input Polygons belong to which output Polygons; internal boundaries are dissolved using the rgeos package gUnaryUnion function. If the rgeos package is not available, and if the gpclib package is available and the user confirms that its restrictive license conditions are met, its union function will be used.

unionSpatialPolygons(SpP, IDs, threshold=NULL, avoidGEOS=FALSE, avoidUnaryUnion=FALSE)



A SpatialPolygons object as defined in package sp


A vector defining the output Polygons objects, equal in length to the length of the polygons slot of SpRs; it may be character, integer, or factor (try table(factor(IDs)) for a sanity check). It may contain NA values for input objects not included in the union


if not NULL, an area measure below which slivers will be discarded (some polygons have non-identical boundaries, for instance along rivers, generating slivers on union which are artefacts, not real sub-polygons)


default FALSE; if TRUE force use of gpclib even when GEOS is available


avoid gUnaryUnion if it is available; not relevant before GEOS 3.3.0


Returns an aggregated SpatialPolygons object named with the aggregated IDs values in their sorting order; see the ID values of the output object to view the order.


When using GEOS Unary Union, it has been found that some polygons are not dissolved correctly when the absolute values of the coordinates are very small. No work-around is available at present.


Roger Bivand


if (rgeosStatus()) { nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1], proj4string=CRS("+proj=longlat +datum=NAD27")) lps <- coordinates(nc1) ID <- cut(lps[,1], quantile(lps[,1]), include.lowest=TRUE) reg4 <- unionSpatialPolygons(nc1, ID) row.names(reg4) }
#> Warning: readShapePoly is deprecated; use rgdal::readOGR or sf::st_read
#> [1] "(-77.8,-75.9]" "(-79.3,-77.8]" "(-81.2,-79.3]" "[-84.1,-81.2]"