用离散色标绘制填充轮廓图

Question

Thanks to this answer I've been able to plot maps from NetCDF files with the correct projection and overlay map. However, when plotting filled contour maps using ggplot, I encounter an error. (see the linked answer for the input file)

#See https://gis.stackexchange.com/questions/120900/plotting-netcdf-file-using-lat-and-lon-contained-in-variables

# First, estimate the extent.
# We start with the lat and long, then project it to the Lambert Conformal projection
library(raster)
inputfile <- "file.nc"

# Grab the lat and lon from the data
lat <- raster(inputfile, varname="xlat")
lon <- raster(inputfile, varname="xlon")

# Convert to points and match the lat and lons
plat <- rasterToPoints(lat)
plon <- rasterToPoints(lon)
lonlat <- cbind(plon[,3], plat[,3])

# Specify the lonlat as spatial points with projection as long/lat
lonlat <- SpatialPoints(lonlat, proj4string = CRS("+proj=longlat +datum=WGS84"))

# Need the rgdal package to project it to the original coordinate system
library("rgdal")

# My best guess at the proj4 string from the information given
mycrs <- CRS("+proj=lcc +lat_1=35.00 +lat_2=51.00 +lat_0=39.00 +lon_0=14.00 +x_0=-6000. +y_0=-6000. +ellps=sphere +a=6371229. +b=6371229. +units=m +no_defs")
plonlat <- spTransform(lonlat, CRSobj = mycrs)
# Take a look
plonlat
extent(plonlat)

# Yay! Now we can properly set the coordinate information for the raster
pr <- raster(inputfile, varname="pr")
# Fix the projection and extent
projection(pr) <- mycrs
extent(pr) <- extent(plonlat)
# Take a look
pr
plot(pr)

# Project to long lat grid
r <- projectRaster(pr, crs=CRS("+proj=longlat +datum=WGS84"))

library(rasterVis)
#Plot with ggplot:
#Add an overlay map
library(maps)
world    <-data.frame(map(plot=FALSE)[c("x","y")])
gplot(r*86400) + 
  geom_tile(aes(fill=value)) + 
  scale_fill_discrete() + 
  geom_path(aes(x,y), data=world) + 
  coord_equal(xlim=c(xmin(r), xmax(r)), ylim=c(ymin(r), ymax(r)))

Error: Continuous value supplied to discrete scale
This is because I want to plot a discrete contour map, with discrete colours, not with a continuous colour scale. Of course it works if using scale_fill_continuous.
I cannot use fill=factor(value) beacuse it will take ages (and GBs of memory) and then return me with a color entry for each single different value in the dataset. I could manually create some "bins" and modify the dataset to fit into those, but I feel this would be a workaround for a much easier solution.

What is the simple elegant solution I'm missing? Shouldn't I use geom_tile (or geom_raster)?

EDIT: Thsi is a pdf example of what I'd like to get: https://copy.com/yMDzEt4i1ELMxpca That plot is created with GrADS. The presence or absence of a color legend is not important, sometimes I use it sometimes I don't.

EDIT2: cut(value) could do but as stated I'd like a ggplot solution and additionally...

...is an example of what it produces. I'd like the labels to be between the colors, not above, like this:

Answer 1

Based on your edits I would do the following:

r$value.bin <- cut(r$value, c(0:4, 2 * (3:5)))
gplot(r*86400) + 
    geom_tile(aes(fill = value.bin)) + 
    scale_fill_manual(
       values = colorRampPalette(brewer.pal(9, "Greens"))(nlevels(r$value.bin))) + 
    geom_path(aes(x,y), data=world) + 
    coord_equal(xlim=c(xmin(r), xmax(r)), ylim=c(ymin(r), ymax(r)))

Maybe you have to play with the colours in scale_fill_manual and the cutting a bit such that there are spots with an NA color.