Add single points to levelplot

Question

I am trying to help a friend with a plot, but end up getting stuck. He wants to plot sample locations on a levelplot. The raster is from the package unmarked. (data(Switzerland)). I have tried it with a dummy dataframe, where I randomly chose some locations.

I have found the following question Add XY points to raster map generated by levelplot which is more or less what I would like to do.

So I have tried the following code:

x <- c("980000", "1100000", "1200000")
y <- c("120000", "170000", "100000")
name <- c("a", "b", "c")
dummy <- as.data.frame(cbind(x, y, name))

levelplot(elevation ~ x + y, Switzerland, aspect="iso",col.regions=terrain.colors(100)) +
  layer(sp.points(dummy, cex=2, col=1))

But I end up with an error message

Error in .local(obj, ...) : any(sp) is not TRUE

I tried to understand what sort of input sp.points() needs and what I'm doing wrong, but failed.

Answer 1

Convert dummy to an sp-object, i.e., a SpatialPointsDataFrame in your case:

R> library("sp")
R> dummy <- data.frame(x = as.numeric(x), y = as.numeric(y), name = name)
R> coordinates(dummy) <- ~ x + y
R> class(dummy)
[1] "SpatialPointsDataFrame"
attr(,"package")
[1] "sp"

Reproducible example:

library("sp")
library("lattice")
library("latticeExtra")
library("unmarked")

data(Switzerland)

x <- c(980000, 1100000, 1200000)
y <- c(120000, 170000, 100000)
name <- c("a", "b", "c")

dummy <- data.frame(x, y, name)
coordinates(dummy) <- ~ x + y


p <- levelplot(elevation ~ x + y, Switzerland,
               aspect = "iso", col.regions = terrain.colors(100)) +
     layer(sp.points(dummy, cex = 2, col = 1))
print(p)