sphere01.r

require(rgl)

# Download a topographic image of Earth from NASA's website
# The file downloaded here is actually the lowest available
# resolution. Higher resolutions are available.
download.file("http://eoimages.gsfc.nasa.gov/images/imagerecords/73000/73751/world.topo.bathy.200407.3x5400x2700.jpg", "world.topo.jpg", mode = "wb")

# Use third party software (ImageMagick) to convert the jpeg
# into a png file (rgl can only work with png).
# I also reduce the size and number of colours to achieve an
# acceptable file size
shell("convert world.topo.jpg -resize 50% -colors 64 world.topo.png")

# function to convert polar to Cartesian coordinates,
# assuming that the world is a perfect sphere
polar_to_cart <- function(long, lat, alt = 1)
{
  x <- outer(long*pi/180, lat*pi/180, function(x, y) alt*cos(y)*cos(x))
  y <- outer(long*pi/180, lat*pi/180, function(x, y) alt*cos(y)*sin(x))
  z <- outer(long*pi/180, lat*pi/180, function(x, y) alt*sin(y))
  return (list(x = x, y = y, z = z))
}

# Specify reference coordinates to project our image onto
lat  <- seq(90,-90, len=50)
long <- seq(-180, 180, len=50)

# get Cartesian coordinates from the specified longitudes and latitudes
world_coords <- polar_to_cart(long, lat)