How to read GRIB files with R / GDAL? - r

Question

I'm trying to read a GRIB file wavedata.grib with wave heights from the ECMWF ERA-40 website, using an R function. Here is my source code until now:
mylat = 43.75
mylong = 331.25
# read the GRIB file
library(rgdal)
library(sp)
gribfile<-"wavedata.grib"
grib <- readGDAL(gribfile)
summary = GDALinfo(gribfile,silent=TRUE)
save(summary, file="summary.txt",ascii = TRUE)
# >names(summary): rows columns bands ll.x ll.y res.x res.y oblique.x oblique.y
rows = summary[["rows"]]
columns = summary[["columns"]]
bands = summary[["bands"]]
# z=geometry(grib)
# Grid topology:
# cellcentre.offset cellsize cells.dim
# x 326.25 2.5 13
# y 28.75 2.5 7
# SpatialPoints:
# x y
# [1,] 326.25 43.75
# [2,] 328.75 43.75
# [3,] 331.25 43.75
myframe<-t(data.frame(grib))
# myframe[bands+1,3]=331.25 myframe[bands+2,3]=43.75
# myframe[1,3]=2.162918 myframe[2,3]=2.427078 myframe[3,3]=2.211989
# These values should match the values read by Degrib (see below)
# degrib.exe wavedata.grib -P -pnt 43.75,331.25 -Interp 1 > wavedata.txt
# element, unit, refTime, validTime, (43.750000,331.250000)
# SWH, [m], 195709010000, 195709010000, 2.147
# SWH, [m], 195709020000, 195709020000, 2.159
# SWH, [m], 195709030000, 195709030000, 1.931
lines = rows * columns
mycol = 0
for (i in 1:lines) {
if (mylat==myframe[bands+2,i] & mylong==myframe[bands+1,i]) {mycol = i+1}
}
# notice mycol = i+1 in order to get values in column to the right
myvector <- as.numeric(myframe[,mycol])
sink("output.txt")
cat("lat:",myframe[bands+2,mycol],"long:",myframe[bands+1,mycol],"\n")
for (i in 1:bands) { cat(myvector[i],"\n") }
sink()
The wavedata.grib file has grided SWH values, in the period 1957-09-01 to 2002-08-31. Each band refers to a pair of lat/long and has a series of 16346 SWH values at 00h of each day (1 band = 16346 values at a certain lat/long).
myframe has dimensions 16438 x 91. Notice 91 = 7rows x 13columns. And the number 16438 is almost equal to number of bands. The additional 2 rows/bands are long and lat values, all other columns should be wave heights corresponding to the 16436 bands.
The problem is I want to extract SWH (wave heights) at lat/long = 43.75,331.25, but they don't match the values I get reading the file with Degrib utility at this same lat/long.
Also, the correct values I want (2.147, 2.159, 1.931, ...) are in column 4 and not column 3 of myframe, even though myframe[16438,3]=43.75 (lat) and myframe[16437,3]=331.25 (long). Why is this? I would like to know to which lat/long do myframe[i,j] values actually correspond to or if there is some data import error in the process. I'm assuming Degrib has no errors.
Is there any R routine to easily interpolate values in a matrix if I want to extract values between grid points? More generally, I need help in writing an effective R function to extract wave heights like this:
SWH <- function (latitude, longitude, date/time)
Please help.