Merging multiple SpatialPolygonDataFrames in R when number of columns do not match

Question

I would like to merge multiple SpatialPolygonDataFrames into one. For Belgium & the Netherlands this works OK using maptools' spRbind:

td <- tempdir()
bel <- getData("GADM", country="BEL", level=1, path=td)
nld <- getData("GADM", country="NLD", level=1, path=td)
row.names(nld) <- paste("nld", row.names(nld), sep="_")
row.names(bel) <- paste("bel", row.names(bel), sep="_")
library(maptools)
bel_nld <- spRbind(bel, nld)
plot(bel_nld)

However, in my case I would like to merge the province outlines for both Belgium & the Netherlands, ie

td <- tempdir()
bel <- getData("GADM", country="BEL", level=2, path=td)
nld <- getData("GADM", country="NLD", level=1, path=td)
row.names(nld) <- paste("nld", row.names(nld), sep="_")
row.names(bel) <- paste("bel", row.names(bel), sep="_")
library(maptools)
bel_nld <- spRbind(bel, nld)
plot(bel_nld)

but then I get the error message "Error in rbind(deparse.level, ...) : numbers of columns of arguments do not match"

Would anybody know how to resolve this, and obtain the provincial outlines of Belgium & the Netherlands in one SpatialPolygonDataFrame?

Answer 1

Drop all the attributes except the unique name. Keep other ones if you want, just make sure you have the same number in each:

> bel = bel[,"NAME_2"]
> nld = nld[,"NAME_1"]

You can't spRbind if the names are different so make them the same:

> names(bel)="NAME"
> names(nld)="NAME"

If you've kept the country code as well in the first step then names(bel)=c("COUNTRY","NAME") might be needed.

Once that's done:

> belnld = spRbind(bel, nld)

If you want to keep all the attributes of each country, then you probably need to construct two spdfs with all the attributes, filling in NA where one column doesn't exist in the other data set.