Aggregate Polygons in a SpatialPolygons object
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 gpclib package union() function. If the rgeos package is available,
unionSpatialPolygonsGEOS will be used with partial topology preservation instead of the R implementation here by passing arguments through.
unionSpatialPolygons(SpP, IDs, threshold=NULL, avoidGEOS=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)
- 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
- 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.
gpclibPermit() require(gpclib) nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools"), proj4string=CRS("+proj=longlat +datum=NAD27")) lps <- coordinates(nc1) ID <- cut(lps[,1], quantile(lps[,1]), include.lowest=TRUE) reg4 <- unionSpatialPolygons(nc1, ID) sapply(slot(reg4, "polygons"), function(i) slot(i, "ID"))