[R-sig-Geo] how to use levelplot() with a geographic projection

[Waichler, Scott R]

Hi again,

In working on this some more I discovered that I need to transpose a data matrix, then flip the raster object created from the matrix to get the proper rendering in a plot.  Can someone explain why that is?

My example has to do with the state of California, with 2918 1/8-degree cells.  I regret I can't provide a small working example--I don't know to make one up that would clearly show correct vs. incorrect geographic rendering.

library(maps)
library(maptools)
library(raster)
library(rasterVis)
data(stateMapEnv)
state.map <- map("state", plot=F)

# longitude and latitude are vectors giving the respective coordinates 
# for cells that are non-NA (n=2918)

xmn <- min(longitude); xmx <- max(longitude)
ymn <- min(latitude);  ymx <- max(latitude)
x.bounds <- seq(xmn - 1/16, xmx + 1/16, by = 1/8)  # grid spacing is 1/8 degree
y.bounds <- seq(ymn - 1/16, ymx + 1/16, by = 1/8)
ii <- findInterval(longitude, x.bounds)  # indices
jj <- findInterval(latitude, y.bounds)   # indices
index.matrix <- cbind(ii,jj)
num.long <- length(x.bounds) - 1
num.lat <- length(y.bounds) - 1
mat <- array(NA, dim=c(num.long, num.lat)) # full, enclosing array with rows=long and columns=lat
mat[index.matrix] <- runif(n=length(longitude))  # insert non-NA values in the right cells

# Why do I need to transpose the matrix then flip the raster for proper rendering in levelplot?
myrast <- flip(raster(t(mat), xmn=xmn, xmx=xmx, ymn=ymn, ymx=ymx, 
                      crs="+proj=longlat +datum=WGS84"), 
               direction="y")

ext <- as.vector(extent(myrast))
boundaries <- map('state', fill=TRUE, xlim=ext[1:2], ylim=ext[3:4], plot=FALSE)
IDs <- sapply(strsplit(boundaries$names, ":"), function(x) x[1])
bPols <- map2SpatialPolygons(boundaries, IDs=IDs, proj4string=CRS(projection(myrast)))

levelplot(myrast,
          panel = function(...) {
              panel.levelplot(...)
              sp.polygons(bPols)
          },
          margin=F)

--Scott Waichler