Loop polygon clipping with gIntersection - r

Question

Question
I am trying to perform a number of polygon clips using the gIntersection function with R in a loop. I can obtain the correct clips and re-enter data manually (so I can turn the resulting SpatialPolygons object back into a SpatialPolygonsDataFrame object). What I can't do is get this working in a loop with for() or apply().
At the moment this isn't a problem. I have 9 English regions (with London), so it's not a huge challenge to set each clip up manually. But, I want to eventually clip LSOAs in LADs, which essentially means setting up >400 clips.
So, my question is, how do I turn my manual clips into a working loop?
Minimal Reproducible Example
To keep things simple, let's use the English regions (n = 9). For each of the 9 regions, I'm going to clip the counties. The following code loads the appropriate shapefiles and reprojects them as British National Grid:
require(rgdal)
require(rgeos)
# English counties shapefile (~ 10MB zipped)
download.file(
"https://dl.dropboxusercontent.com/s/6o0mi28pjo1kh9k/england-counties.zip",
"ec", method = "wget")
unzip("ec")
ec <- readOGR("england-counties", "england_ct_2011")
proj4string(ec) <- CRS("+init=epsg:27700")
# English regions (~6MB zipped)
download.file(
"https://dl.dropboxusercontent.com/s/p69m0vk2fh4xe3o/england-regions-2011.zip",
"er", method = "wget")
unzip("er")
er <- readOGR("england-regions-2011", "England_gor_2011")
proj4string(er) <- CRS("+init=epsg:27700")
You should be left with two objects, er (English regions) and ec (English counties). Both are SpatialPolygonsDataFrame objects.
Taking the first region - East of England E12000006 - let's clip the counties and turn the result back in to a SpatialPolygonsDataFrame object:
ee <- gIntersection(ec, er[er$CODE == "E12000006", ],
byid = T, drop_not_poly = T)
row.names(ee) <- as.character(gsub(" 0", "", row.names(ee)))
# gIntersection adds ' 0' to each row.name?
ee <- SpatialPolygonsDataFrame(ee, ec#data[row.names(ee), ])
A plot of ee confirms this worked:
As you can see, this is a nice workflow for just a few shapes, but I really want to loop through all regions and, ultimately, many more polygons.
What I've Tried
I'm not very good with apply() loops, so what I've tried so far is a for() loop (which I know is relatively slow, but still quicker than typing everything out!):
regions <- as.character(er$CODE) # length = 9 as expected
for(i in 1:length(regions)){
as.name(paste0(regions[i], "c")) <-
gIntersection(ec, er[er$CODE == regions[1], ], byid = T, drop_not_poly = T)
}
Rather than the expected behaviour I get the following error:
Error in as.name(paste0(regions[1], "c")) <- gIntersection(ec, er[er$CODE == :
could not find function "as.name<-"
I also tried wrapping the object name in an eval() but get the following error:
Error in eval(as.name(paste0(regions[1], "c"))) <- gIntersection(ec, er[er$CODE == :
could not find function "eval<-"
What am I missing?
In addition to the gIntersection, I would like to re-create a SpatialPolygonsDataFrame object if possible. I've tried the following code, having done one gIntersection manually, but again it doesn't work:
for(i in 1:length(regions)){
row.names(as.name(paste0(regions[i], "c"))) <- as.character(gsub(" 0", "",
row.names(as.name(paste0(regions[i], "c")))))
}
I get the following error:
Error in `rownames<-`(x, value) :
attempt to set 'rownames' on an object with no dimensions
I'm also not sure how to increment the " 0", as this increases by one for each new region (" 1", " 2", etc.)
Again, setting the first example up manually I also can't perform the final SpatialPolygonsDataFrame step:
for(i in 1:length(regions)){
as.name(regions[i]) <- SpatialPolygonsDataFrame(regions[i],
ec#data[row.names(regions[i], )])
}
For this I get the following error:
Error in stopifnot(length(Sr#polygons) == nrow(data)) :
trying to get slot "polygons" from an object of a basic class ("character") with no
slots
Where I've looked
The following SO examples are related by do not seem to help, or at least I can't see how I would make them apply to this example:
rgeos gIntersection in loop takes too long to clip path network
How to clip WorldMap with polygon in R?
https://gis.stackexchange.com/questions/33278/no-intersection-found-between-polygons-i-know-intersect
Thanks for taking the time to read this.

Answer 1

Does this help?
ee <- lapply(regions, function(x)
gIntersection(ec, er[er$CODE == x, ], byid = TRUE, drop_not_poly = TRUE))
This gives you a list of SpatialPolygonsDataFrames, one for each region. Which you can access in the usual way, e.g.
ee[[1]]
plot(ee[[1]]) # to plot the first region with counties
Edit
Your orignial code should work with a sligh modification (see blow).
res <- list()
for (i in 1:length(regions)) {
ee <- gIntersection(ec, er[er$CODE == regions[i], ],
byid = TRUE, drop_not_poly = TRUE)
row.names(ee) <- as.character(gsub(paste0(" ", i-1), "", row.names(ee)))
ee <- SpatialPolygonsDataFrame(ee, ec#data[row.names(ee), ])
res[[i]] <- ee
}
If that solves the problem, then the problem was, that row names of ee always incremented by one and you did not account for this.