[R-sig-Geo] ECMWF grib and raster

[bart]

Hi All

I'm trying to read grib files using Raster, form the ECMWF interim
dataset (example file linked below). I don't get errors but the
coordinate system of the read raster does not seem to match.

  > raster("tmp.grib")
class       : RasterLayer
filename    : /home/bart/Desktop/tmp.grib
nrow        : 121
ncol        : 240
ncell       : 29040
min value   : ?
max value   : ?
projection  : +proj=longlat +a=6367470 +b=6367470 +no_defs
extent      : -180, 180, -91.5, 90  (xmin, xmax, ymin, ymax)
resolution  : 1.5, 1.5  (x, y)


Notice the minimal latitude outside this world. For comparison i
extracted the same data using the grib_get_data program with the
following command: grib_get_data tmp.grib > bla.tab (small manual format
conversion for reading in R). grib_get_data is provided by:
libgrib-api-tools (http://packages.ubuntu.com/maverick/libgrib-api-tools) .

Comparing the datasets gives the following results
  > summary(b<-read.csv("bla.tab", sep=","))
      Latitude     Longitude           Value
   Min.   :-90   Min.   :-180.00   Min.   :2.093
   1st Qu.:-45   1st Qu.: -90.38   1st Qu.:2.695
   Median :  0   Median :  -0.75   Median :2.836
   Mean   :  0   Mean   :  -0.75   Mean   :2.791
   3rd Qu.: 45   3rd Qu.:  88.88   3rd Qu.:2.969
   Max.   : 90   Max.   : 178.50   Max.   :3.147
  > summary(a<-rasterToPoints(raster("tmp.grib")))
         x                 y                V3
   Min.   :-179.25   Min.   :-90.75   Min.   :209.3
   1st Qu.: -89.62   1st Qu.:-45.75   1st Qu.:269.5
   Median :   0.00   Median : -0.75   Median :283.6
   Mean   :   0.00   Mean   : -0.75   Mean   :279.1
   3rd Qu.:  89.62   3rd Qu.: 44.25   3rd Qu.:296.9
   Max.   : 179.25   Max.   : 89.25   Max.   :314.7

It seems the raster grid is offset by half a grid cell size. I'm not
sure what the grib file format specification is but since
libgrib-api-tools seems to be provided by ECMWF i tend to believe their
correct.


Bart

ps tmp.grib is downloadable here: 
https://depot.uni-konstanz.de/cgi-bin/exchange.pl?g=nv3xd (less than 100kb)