readGDAL() and HDF5 files
Hi Sebastian,
When you say the geographic coordinates are not a regular grid - is it
that the actual grid is Mercator but the NetCDF file stores an X and Y
vector separately for each unique longitude and latitude? I've seen this
many times, but never with enough metadata to determine that without
guessing. I've seen some documents that refer to the source grid as
being in Mercator, but never any that mention explicitly the method used
to generate the NetCDF file from those.
If this is the case for your data, I've had success by figuring out an
offset/scale value that work sufficiently by assuming a Mercator grid
Specifically this one but sometimes with an extra X offset to overcome
hemisphere shift.
CRSargs(CRS("+proj=merc"))
I don't have the details available today, but I can dig them up if that
sounds promising. Also, I'd be interested to hear any details you have
about the grids you are using, whether they use this Mercator-kludge or
not.
Cheers, Mike.
Sebastian P. Luque wrote:
On Fri, 23 Jan 2009 11:01:21 +0100 (CET), Roger Bivand <Roger.Bivand at nhh.no> wrote:
On Fri, 16 Jan 2009, Sebastian P. Luque wrote:
Hi,
I'm trying to use HDF5 files with this structure:
Sorry for the delay. Having now set up a reprodicable route (OSGeo4W
GDAL and drivers, rgdal built against OSGeo4W as in a posting a couple
of days ago), I can see the problem, but have no resolution.
Doing gdalinfo on the data itself, I see geographical coordinates, not
projected. GDAL does not retrieve the grid metadata correctly, and
assigns default 1x1 grid cells starting at c(0.5, 0.5). Using
gdal_translate to convert it to a GTiff, I see that I have to set p4s
to NA to read at all, and the problems remain. I would be interested
in knowing who or what wrote the HDF5 file, because it doesn't seem to
be what it says it is (no grid metadata, different CRS from your
assumption), and so the data may actually be in the wrong order too.
Thanks for looking into this Roger. The data are distributed by the OSI-SAF group (http://www.osi-saf.org). I was able to work with the data using the NetCDF files they distribute (for a subset of their data for now), using NetCDF tools outside R. However, I'd like to do it more directly from R. In the NetCDF files, longitude, latitude, and the data, are stored in separate data subsets. The grid is produced using the CRS specification I provided (from OSI-SAF's documentation), however I cannot see where the actual coordinates in that projection are in the files. The geographical coordinates are the inverse-projected coordinates of the grid, hence not a regular grid. I might ask in the GDAL list for more help. Cheers, ------------------------------------------------------------------------ No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.176 / Virus Database: 270.10.12/1910 - Release Date: 22/01/2009 6:28 PM