Plotting netcdf in R with correct grid

Question

My goal is to plot nitrate (no3) data on a world map, using the correct longitude and latitude for these data.

There are two netcdf files:
1. with the data
2. wi

Answer 1

library(ncdf)
data <- open.ncdf(file1)
no3 <- get.var.ncdf(data,varid="no3")
sigma_plot <- no3[,,20]
grid <- open.ncdf(file2)
plon <- get.var.ncdf(grid,varid="plon")
plat <- get.var.ncdf(grid,varid="plat")

Contrary to what I previously understood, each cell of sigma_plot corresponds to one cell of plon and plat.

A <- array(c(plon,plat,a),dim=c(180,193,3))
B <- apply(A, 3, cbind)

Now B is an array containing for each row: (longitude, latitude, value). But it is not a regular grid, so you need to interpolate a regular grid. Easiest way would be using interp from package akima:

library(akima)
C <- interp(B[,1],B[,2],B[,3], 
            xo=seq(-180,180,1),yo=seq(-90,90,by=1), #you can tweak here the resolution
            duplicate='mean') #for some reasons some entries are duplicates, i don t know how you want to handle it.

image(C) #for instance, or filled.contour if you prefer
library(maptools)
data(wrld_simpl)
plot(wrld_simpl, add=TRUE, col="white") #To add a simple world map on top