[Gdal-dev] matching metadata for different file formats

[Kolberg Sjur A]

Hello, Matt,

It seems that even your GRIB data hasn't got the Polar Stereographic info.

I have just been working with the GRIB driver, and I know that there is a new version coming which may catch some more projection info. The new version is not included in the latest GDAL (1.4.2) or FWtools (1.3.2) releases, but maybe the sources are in place.

Also, there is a hatch with data projected on a spherical Earth. People sometimes forget that the ellipsiod and datum are not only properties of the projection (from a curved to plane surface), but of the latlong coordinate system itself. Meaning; The geographical position 10degE, 60degN is not at the same place as 10degE, 60degN in spherical-earth degrees. The difference at 60 deg N is around 18km in N-S direction. You need to figure out if this is the case for your data. See http://lists.maptools.org/pipermail/proj/2005-March/001520.html

Regards,

Sjur K :-)


Sjur Kolberg
SINTEF Energiforskning A/S
Sem Sælands vei 11
7465 Trondheim
tlf. 73 59 72 78
fax  73 59 72 50 

> -----Original Message-----
> From: gdal-dev-bounces at lists.maptools.org 
> [mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Matt Fearon
> Sent: 28 juni 2007 21:48
> To: gdal-dev at lists.maptools.org
> Subject: [Gdal-dev] matching metadata for different file formats
> 
> Dear GDAL users:
> 
> I am converting grib files to netcdf as a 1st step using ncar 
> command language. netcdf is an easier format for me in the 
> long run for data storage. After this 1st step, I would like 
> to reprojected my data from polar stereographic to 
> geographic. The datum of the polar stereographic is unique in 
> that data are mapped to a sphere with radii 6371200 km. 
> Here is my trouble:
> 
> If I skip the 1st step and simply reproject the grib data to 
> geographic using gdalwarp everything works perfectly. 
> However, if try to reproject the data after the 1st step, I 
> am unsuccessful. I am also performing some simple 
> calculations on the data before going from grib to netcdf, 
> but I am not changing the coordinate system, projection, 
> datum etc...Some of the metadata is lost going from grib to 
> netcdf, and therefore, I apply gdal_translate to the netcdf 
> file adding the lost metadata back in and converting the 
> format to geotiff. However, after these steps, I still notice 
> some slight differences in the metadata between the original 
> grib and the geotiff. I am struggling to make the metadata 
> consistent for 3 parameters (GEOGCS, DATUM, and SPHEROID), 
> please see metadata comparison at the base of this email from 
> gdalinfo.
> 
> Any suggestions on how I can make these 3 parameters 
> consistent between files would be much appreciated. Perhaps 
> some combination of options using gdal_translate? Although, I 
> have tried many combinations unsuccessfully, and therefore, 
> the data do not map correctly. gdalinfo may be excluding some 
> information as well. However, everything is consistent with 
> the metadata except these 3 parameters. My original data are 
> hourly NEXRAD precipitation, mosaicked over the CONUS at 4km, 
> in grib format, polar stereographic projection assuming the 
> Earth as a sphere with radii of 6371200 km, downloaded from 
> NCEP. I am converting hourly to daily and storing in netcdf 
> for various projects.
> 
> thanks in advance for the help,
> Matt
> 
> 
> Driver: GRIB/GRIdded Binary (.grb)
> Files: gag4.19980201.01
> Size is 1160, 880
> Coordinate System is:
> PROJCS["unnamed",
>   GEOGCS["Coordinate System imported from GRIB file",
>       DATUM["unknown",
>           SPHEROID["Sphere",6371200,0]],
> 
> Driver: GTiff/GeoTIFF
> Files: 19980201.tif
> Size is 1160, 880
> Coordinate System is:
> PROJCS["unnamed",
>   GEOGCS["unnamed ellipse",
>       DATUM["unknown",
>           SPHEROID["unnamed",6371200,0]], 
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