Merging polygons without dissolving adjacent boundaries
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
# See http://stackoverflow.com/questions/29310962/variant-of-rgeosgunion-that-wont-dissolve-adjacent-polygons | |
# for the relevant problem formulation. | |
library(sp) | |
unit_rect <- cbind( | |
x = c(0, 0, 1, 1, 0), | |
y = c(0, 1, 1, 0, 0)) | |
translate <- function (spobj, delta = c(0, 0)) { | |
Polygon(t(t(coordinates(spobj)) + delta)) | |
} | |
ring1 <- Polygon(coords = unit_rect) | |
hole1 <- Polygon(coords = t(0.25 + 0.5 * t(unit_rect)), hole = TRUE) | |
poly1 <- Polygons(list("ring" = ring1, "hole" = hole1), ID = "poly1") | |
ring2 <- translate(ring1, delta = c(1, 0)) | |
hole2 <- translate(hole1, delta = c(1, 0)) | |
poly2 <- Polygons(list("ring" = ring2, "hole" = hole2), ID = "poly2") | |
ring3 <- translate(ring1, delta = c(0, 1)) | |
hole3 <- translate(hole1, delta = c(0, 1)) | |
poly3 <- Polygons(list("ring" = ring3, "hole" = hole3), ID = "poly3") | |
spobj_A <- SpatialPolygons(list(poly1, poly2)) | |
spobj_B <- SpatialPolygons(list(poly3)) | |
# Have a peek. | |
plot(spobj_A) | |
plot(spobj_B, add = TRUE) | |
# Not the desired outcome! Shared boundaries are dissolved. | |
unioned <- rgeos::gUnion(spobj_A, spobj_B) | |
plot(unioned) | |
# First case: merging two Polygons objects. | |
setMethod( | |
merge, | |
signature = c("Polygons", "Polygons"), | |
definition = function (x, y, ID, ...) { | |
# TODO: better automatic ID renaming. | |
if (missing(ID)) ID <- paste(x@ID, y@ID, ".") | |
Polygons(append(x@Polygons, y@Polygons), ID) | |
}) | |
test1 <- SpatialPolygons(list(merge(poly1, poly2, ID = "A"))) | |
plot(test1) | |
# Second case: merging two SpatialPolygons objects. | |
setMethod( | |
merge, | |
signature = c("SpatialPolygons", "SpatialPolygons"), | |
definition = function (x, y, IDs, ...) { | |
SpatialPolygons(append( | |
Reduce(merge, x@polygons), | |
Reduce(merge, y@polygons))) | |
}) | |
test2 <- merge( | |
spChFIDs(spobj_A, c("A.1", "A.2")), # TODO: better automatic ID renaming. | |
spChFIDs(spobj_B, "B.1")) | |
plot(test2) |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment