How to find out the map scale of a SpatialPolygons object in R?

Question

I would like to find out the exact map scale in m or km of my SpatialPolygons object and plot it on my spplot so that the reader gets an idea about the distances. That means that I would like to find out what number to put in for the ??? in the plot.

library("sp")
library("CARBayes")

data("spatialhousedata")
proj4string(spatialhousedata) <- CRS("+proj=cass +lat_0=52.41864827777778 +lon_0=13.62720366666667 
                                      +x_0=40000 +y_0=10000 +datum=potsdam +units=m +no_defs 
                                      +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 ")

scalebar=list("SpatialPolygonsRescale", layout.scale.bar(),
              offset = c(225000,647000), scale = 10000, fill=c("transparent","black"))
text1=list("sp.text",  c(225000,649000), "0")
text2=list("sp.text",  c(230000,649000), "??? m")

spplot(spatialhousedata, "price",
       sp.layout=list(scalebar, text1, text2))

Maybe something with sp::coordinates or Imap::gdist? Any prior reprojection of the SpatialPolygons necessary to find this out?

Answer 1

Your data are projected (try is.projected(spatialhousedata)); I would go for the

text2=list("sp.text",  c(235000,649000), "10 km")

the scale bar can be confirmed by drawing the axis scales:

spplot(spatialhousedata, "price", scales = list(draw=TRUE), 
    sp.layout=list(scalebar, text1, text2))