source
The ETOPO5 dataset was downloaded in late 2009 from the NOAA
  website, and decimated from 1/12th degree resolution to 1/2 degree
  resolution as follows:
  library(oce)
f <- file("etopo5.dat", "rb")
nx <- 12 * 360
ny <- 12 * 180
d <- t(matrix(readBin(f, "integer", size=2, endian="big", n=nx*ny), 
              byrow=TRUE, nrow=ny))
z <- d[seq(1, nx, by=6), seq(ny, 1, by=-6)]
topoWorld  <- as.topo(seq(0.25,359.75,1/2),seq(-89.75,89.75,1/2),z)