[R-sig-Geo] inconsistencies between readGDAL and other software (GrADS) when reading GRIB1 files

[Roger Bivand]

On Thu, 17 Nov 2011, Ariel Ortiz-Bobea wrote:

> Hi,
>
> I've been importing GRIB1 files into R through the readGDAL command in the
> GDAL package.

The readGDAL() function in the rgdal package uses GDAL and its drivers, so 
see:

http://www.gdal.org/frmt_grib.html

for details. Note also:

https://stat.ethz.ch/pipermail/r-sig-geo/2011-January/010760.html
http://grass.osgeo.org/wiki/GRIB

for a bug in the GDAL driver and a workaround, and other GRIB threads on 
this list.

>
> However, the GRIB1 files are multi-band and during the import process I
> "lose" the variable names and get generic names: band1, band2, etc.
>

As far as I can see, band names are not supported in GDAL in:

http://www.gdal.org/classGDALDataset.html

or

http://www.gdal.org/classGDALRasterBand.html

but may be in http://www.gdal.org/classGDALRasterAttributeTable.html.

Could you please try running:

GDALinfo("mygrib.grb", returnRAT=TRUE)

to see if you get the band names?

Roger

> I have tried to circumvent this by using other software (GrADS) to obtain
> summary statistics for each variable (of which I know the names) and then
> compare these to summary statistics of each variable/band in R. Matching
> summary statistics would allow me to get back the real names.
>
> However, there are discrepancies between summary statistics. More precisely,
> there are differences between minimums, means and maximums between both
> approaches AND these differences are not of the same magnitude (ruling out a
> homogenous shift in values).
>
> Moreover, GrADS gives temperatures in Kelvin and readGDAL seems to be
> transforming them to Celsius BUT the difference between minimum, maximum and
> means is not consistently 273.15 as one would expect. For instance, for soil
> temperature the difference is 271.441 (for the mean), 270.514 (for the max)
> and 273.149 (for the min), but for Dew point it is 269.436 (for the mean),
> 272.58 (for the max) and 273.349 (for the min). So it is not even consistent
> across different temperature variables.
>
> I wonder if this has to do with how GrADS or readGDAL read these GRIB1 files
> (or the way I'm importing this)... would there be any hidden processing
> going on that would explain this? Would using netCDF format files rather
> than GRIB1 help in any way? Any suggestions or comments on how to import
> this data in a manner in a proper way?
>
> Thanks a lot in advance for any guidance on this,
>
> Ariel
>
> Here is my example file:
> http://dl.dropbox.com/u/45311184/US48_merged_AWIP32.1985120100.RS.flx (in
> GRIB1 format)
>
> R code:
> -------
> # import
>   flx<-  readGDAL("1985/US48_merged_AWIP32.1985120100.RS.flx")
> # fix missing values (imported as 9999)
>   for (n in 1:10 ){
>      is.na(flx[[n]]) <- flx[[n]] > 9000
>   }
> # print summary statistics for each variable/band
>   for (n in 1:10 ){
>      print(summary(flx[[n]])[c(4,6,1)])
>   }
>
> GrADS:
> --------
> code: http://dl.dropbox.com/u/45311184/gradscode.txt
> files to be included in the same folder with the .flx file
> http://dl.dropbox.com/u/45311184/US48_merged_AWIP32.1985120100.RS.flx.ctl
> http://dl.dropbox.com/u/45311184/US48_merged_AWIP32.1985120100.RS.flx.idx
>
>
> -----
> Ariel Ortiz-Bobea
> PhD Candidate in Agricultural & Resource Economics
> University of Maryland - College Park
> --
> View this message in context: http://r-sig-geo.2731867.n2.nabble.com/inconsistencies-between-readGDAL-and-other-software-GrADS-when-reading-GRIB1-files-tp7006160p7006160.html
> Sent from the R-sig-geo mailing list archive at Nabble.com.
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no