In R, I have single
SpatialPolygons object (i.e. multi-polygons) containing several hundred polygons. I would like to split this
SpatialPolygons object into a list of
Polygons (i.e. holes should remain attached to the parent polygon).
Any idea how to do this?
Using the following example provided in the
# simple example, from vignette("sp"): Sr1 = Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2))) Sr2 = Polygon(cbind(c(5,4,2,5),c(2,3,2,2))) Sr3 = Polygon(cbind(c(4,4,5,10,4),c(5,3,2,5,5))) Sr4 = Polygon(cbind(c(5,6,6,5,5),c(4,4,3,3,4)), hole = TRUE) Srs1 = Polygons(list(Sr1), "s1") Srs2 = Polygons(list(Sr2), "s2") Srs3 = Polygons(list(Sr3, Sr4), "s3/4") SpP = SpatialPolygons(list(Srs1,Srs2,Srs3), 1:3)
out = lapply(SpP@polygons, slot, "Polygons"). I get a list of three
However, the case I am trying to solve is a bit different from this example. The
SpatialPolygons object I am trying to split is the result of a geometric union done with the
gUnaryUnion function (in the
RGEOS package). If I apply
out <- lapply(merged.polygons@polygons, slot, "Polygons"), I get a unique list of
Polygon objects (n.b. not a list of
Polygons objects). In other words, each polygon is separated from its hole(s).
topol <- sapply(unlist(out), function(x) x@hole)
> length(topol)  4996 > sum(topol, na.rm=TRUE)  469
In order for rgeos to function properly it is necessary that all holes within a given POLYGON or MULTIPOLYGON geometry must belong to a speciﬁc polygon. The SpatialPolygons class implementation does not currently include this information. To work around this limitation rgeos uses an additional comment attribute on the Polygons class that indicates which hole belongs to which polygon. Under the current implementation this comment is a text string of numbers separated by spaces where the order of the numbers corresponds to the order of the Polygon objects in the Polygons slot of the Polygons object. A 0 implies the Polygon object is a polygon, a non-zero number implies that the Polygon object is a hole with the value indicating the index of the Polygon that “owns” the hole.
createSPComment() function in
RGEOS is likely to be a workaround to reaggregate Polygons and holes.