Scaling the scale bar

Dear list members,

I have got the feeling, that I missed something substantial about /layout.scale.bar/.

For example (not a pretty one, but might clarify my point)

URL      <- "http://www.geodatenzentrum.de/auftrag1/archiv/vektor/vg250_ebenen/2012/vg250_2012-01-01.utm32s.shape.ebenen.zip"
td       <- tempdir()
setwd(td)
temp     <- tempfile(fileext = ".zip")
download.file(URL, temp)
unzip(temp)
dir_temp <- paste(tempdir(),

+                   "\\vg250_0101.utm32s.shape.ebenen",
+                   "\\vg250_ebenen",
+                   sep = "")

require(rgdal)
States <- readOGR(dsn      = dir_temp,

+                   layer    = "vg250_bld",
+                   encoding = "UTF8")

Scale  <- list("SpatialPolygonsRescale",

+           layout.scale.bar(),
+           offset = c(mean(States at bbox[1, ]), mean(States at bbox[2, ])),
+           scale  = 0.1 * mean(States at bbox),
+           fill   = c("white", "black"))

spplot(States, "GEN", sp.layout = Scale)

draws a map of Germany?s States. Now, I would like to change the scale bar, so that it shows a distance of 100 kilometers. How can I assure that the length of the scale bar on the map corresponds with 100 kilometers in reality? (It might be too obvious or some other reason I do not get it. It feels already like the dumbest question to myself. So, my apologies in advance.)

Cheers,

Alex

Alexander Sommer
Forschungsverbund Deutsches Jugendinstitut/Technische Universit?t Dortmund