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.
unionSpatialPolygons(SpP, IDs, threshold=NULL)
- 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)
- 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.
library(sp) library(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"))
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