# Reverse clipping (erasing ) in R?

A reverse clip saves only the part of your spatial object that is outside the bounds of another object, as opposed to a regular clip which saves the parts that are inside the other object.

Performing reverse clip in ArcMap? shows how to do it in ArcMap.

How do I do this in R?

Reproducible example (on Linux machines):

``````system("wget 'https://github.com/Robinlovelace/Creating-maps-in-R/archive/master.zip' -P /tmp/")
unzip("/tmp/master.zip", exdir = "/tmp/master")
plot(uk)
plot(lnd, add = T, col = "black")
``````

What I want here to do is to save all of the UK except for London. Visually, I want the black shape in the resulting image to be a hole.

sf package draws on Geometry Engine Open Source, and as such can access the list of commands such as st_within etc.

One such command, st_difference, will do the job:

``````require(sf)

# make a square simple feature
s <- rbind(c(1,1), c(1,5), c(5,5), c(5,1), c(1,1))
s.sf <-st_sfc(st_polygon(list(s)))
s.pol = st_sf(ID = "sq", s.sf)

# make a smaller triangle simple feature
s2 <- rbind(c(2,2), c(3,4), c(4,2), c(2,2))
s2.sf <-st_sfc(st_polygon(list(s2)))
s2.pol = st_sf(ID = "tr", s2.sf)

# find the 'difference', i.e. reverse of st_intersection
t <- st_difference(s.pol,s2.pol)

plot(t)

# have a look at the new geometry, a well known text format with exterior followed by hole
st_geometry(t)[[1]]
POLYGON((1 1, 1 5, 5 5, 5 1, 1 1), (2 2, 4 2, 3 4, 2 2))
``````

can also be done by coercing Sp to sf with st_as_sf. Heed the warnings as attributes can be tricky to manage!

Seems to be a simple application of `gDifference` from the `rgeos` package:

``````> require(rgeos)
> ukhole = gDifference(uk, lnd)
Warning message:
In RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, "rgeos_difference") :
spgeom1 and spgeom2 have different proj4 strings
> plot(ukhole)
``````

The projection warning is because the `LondonBoroughs` shapefile doesn't have a `.prj` file.

Just to make sure that's a hole and not an outline or another solid polygon:

``````> gArea(lnd) + gArea(ukhole) - gArea(uk)
[1] 0
``````

Bit late to the party, but there is a simple way to do this with mask using the 'inverse' argument;

``````ukhole <- mask(uk, lnd, inverse = TRUE)
``````
• From the raster package. And from sf any ideas? – RobinLovelace Aug 3 '17 at 10:39