Background
I'm interested in simplifying polygons with use of the gSimplify
function available through the rgeos
package.
Reproducible example
A reproducible example can be generated with use of the code below:
# Data sourcing -----------------------------------------------------------
# Download an read US state shapefiles
tmp_shps <- tempfile()
tmp_dir <- tempdir()
download.file(
"http://www2.census.gov/geo/tiger/GENZ2014/shp/cb_2014_us_state_20m.zip",
tmp_shps
)
unzip(tmp_shps, exdir = tmp_dir)
# Libs
require(rgdal)
require(rgeos)
# Read
us_shps <- readOGR(dsn = tmp_dir, layer = "cb_2014_us_state_20m")
# Simplified --------------------------------------------------------------
# Simplifiy
us_shps_smpl <- gSimplify(spgeom = us_shps,
tol = 200,
topologyPreserve = TRUE)
Preview
par(mfrow = c(2,1))
plot(us_shps_smpl, main = "Simplified")
plot(us_shps, main = "Original")
Problem
In addittion to simplifying polygons the gSimplify
function changed classes of the resulting object:
>> class(us_shps)
[1] "SpatialPolygonsDataFrame"
attr(,"package")
[1] "sp"
>> class(us_shps_smpl)
[1] "SpatialPolygons"
attr(,"package")
[1] "sp"
>> names(us_shps)
[1] "STATEFP" "STATENS" "AFFGEOID" "GEOID" "STUSPS" "NAME" "LSAD" "ALAND" "AWATER"
>> names(us_shps_smpl)
[1] "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" "10" "11" "12" "13" "14" "15" "16" "17" "18" "19"
[21] "20" "21" "22" "23" "24" "25" "26" "27" "28" "29" "30" "31" "32" "33" "34" "35" "36" "37" "38" "39"
[41] "40" "41" "42" "43" "44" "45" "46" "47" "48" "49" "50" "51"
Questions
How can I safely reattached the data that was initially available in the original object and transform the resulting
SpatialPolygons
object to aSpatialPolygonsDataFrame
I reckon that one approach would simply involve attaching data frame;but this depends on the order of elements not changing. Are there any other better approaches (ideally preserving initial object class)?
The
sf
package is based entirely on data frames, so its geometry manipulations always preserve the data attached to each feature. The package hasn't caught up with all the standard spatial packages in R yet, but it's fairly easy to go back and forth betweensf
andsp
objects when you need more functionality.Here,
st_simplify()
does the work, but you'll need to project your polygons first: