Merging Polygons in Shape Files with Common Tag IDs: unionSpatialPolygons

Question

I am trying to read from a shape file and merge the polygons with a common tag ID.

library(rgdal)
library(maptools)
if (!require(gpclib)) install.packages("gpclib", type="source")
gpclibPermit()

usa <- readOGR(dsn = "./path_to_data/", layer="the_name_of_shape_file")
usaIDs <- usa$segment_ID
isTRUE(gpclibPermitStatus())
usaUnion <- unionSpatialPolygons(usa, usaIDs)

When I try to plot the merged polygons:

for(i in c(1:length(names(usaUnion)))){
  print(i)
  myPol <- usaUnion@polygons[[i]]@Polygons[[1]]@coords
  polygon(myPol, pch = 2, cex = 0.3, col = i)
}

all the merged segments looks fine except those in around Michigan for which the merger happens in a very weird way such that the resulted area for this particular segment, gives only a small polygon as below.

i = 10
usaUnion@polygons[[i]]@Polygons[[1]]@coords

output:

          [,1]     [,2] 
 [1,] -88.62533 48.03317
 [2,] -88.90155 47.96025
 [3,] -89.02862 47.85066
 [4,] -89.13988 47.82408
 [5,] -89.19292 47.84461
 [6,] -89.20179 47.88386
 [7,] -89.15610 47.93923
 [8,] -88.49753 48.17380
 [9,] -88.62533 48.03317

which turned out to be a small northern island:

I suspect the problem is that for some reason the unionSpatialPolygons function does not like geographically separated polygons [left and right side of Michigan], but I could not find a solution to it yet.

Here is the link to input data as you can reproduce.

Answer 1

I think the problem is not with unionSpatialPolygons but with your plot. Specifically, you are plotting only the first 'sub-polygon' for each ID. Run the following to verify what went wrong:

for(i in 1:length(names(usaUnion))){
  print(length(usaUnion@polygons[[i]]@Polygons))
}

For each of these, you took only the first one.

I got a correct polygon join/plot with the following code:

library(rgdal)
library(maptools)
library(plyr)

usa <- readOGR(dsn = "INSERT_YOUR_PATH", layer="light_shape")
# remove NAs
usa <- usa[!is.na(usa$segment_ID), ]
usaIDs <- usa$segment_ID

#get unique colors
set.seed(666)
unique_colors <- sample(grDevices::colors()[grep('gr(a|e)y|white', grDevices::colors(), invert = T)], 15)

colors <- plyr::mapvalues(
  usaIDs, 
  from = as.numeric(sort(as.character(unique(usaIDs)))), #workaround to get correct color order
  to = unique_colors
)
plot(usa, col = colors, main = "Original Map")


usaUnion <- unionSpatialPolygons(usa, usaIDs)
plot(usaUnion, col = unique_colors, main = "Joined Polygons")