[gdal-dev] NetCDF driver improvements (including groups support)

[Joaquim Luis]

Julian,

Thanks for the offering.
One thing, could you please check if with your improvements the bug #5291  
is fixed?

Thanks

Joaquim


> Hi GDAL team,
>
> I've implemented several improvements to the NetCDF driver and I would  
> like to provide them to the community.
> Main goal of the changes is to add full support of NetCDF-4 including  
> groups.
> NetCDF-4 is the future format of ESA Sentinel-3 products (no groups) and  
> NASA Ocean Color team is switching their L2/L3 products to NetCDF-4 with  
> groups (VIIRS has already switched to the new format in December).
> With the changes NASA L2 products geolocation is automatically handled  
> as geolocation arrays and can be reprojected using gdalwarp.
>
> I validated with autotests that nothing is broken in tests netcdf.py  
> (excepting test 13 but see my point 5), netcdf_cf.py and hdf5.py, using  
> NetCDF-3 and 4 libraries.
> I've also tested the new functionalities on various NetCDF-4 files.
> I think the only possible regression could be for marginal cases where a  
> file was seen directly as a dataset and is now seen as multiple  
> subdatasets (for example if a file has only one var in the top group and  
> has nested groups containing variables), but I think this is not very  
> common.
>
> For the moment I have all these changes in local GIT separated commits  
> on the latest gdal-1.11 branch, let me know what changes you want and  
> how can I provide them.
>
> Changes :
>
> 1) Implement full support for NetCDF-4 groups on reading:
>     - explore recursively all nested groups to create the subdatasets  
> list
>     - subdatasets in nested groups use the /group1/group2/.../groupn/var  
> standard
>       NetCDF-4 convention, excepting for variables in the root group  
> which do not
>       have a leading slash for backward compatibility
>     - when accessing a subdataset using NETCDF:$file:$path, the leading  
> slash is optional
>     - global attributes of each nested group are also collected in the  
> GDAL dataset
>       metadata, using the same convention  
> /group1/group2/.../groupn/NC_GLOBAL#attr_name,
>       excepting for the root group which do not have a leading slash for  
> backward compatibility
>     - when searching for a variable containing auxiliary information on  
> the selected subdataset,
>       like coordinate variables or grid_mapping, we now also search in  
> parent groups (using NCDFResolveVar).
>       I now this is something not specified at this time in the CF  
> convention because CF does not know groups,
>       but it seems logical to me to support this: NetCDF-4 specifies  
> that dimensions of a group are
>       shared to its nested groups, so associated coordinate variables  
> could be defined as the same level of its
>       corresponding dimension.
>     - reference to coordinate variables using the "coordinates"  
> attribute support now also absolute paths,
>       this allow for example to specify coordinate variables located  
> outside the group of the selected variable
>       or its parents. Relative paths could be implemented if needed.
>       This feature is used to add support for new NASA Ocean Color L2  
> products.
>
> 2) Implement full read/write support for new NetCDF4 types NC_UBYTE,  
> NC_USHORT, NC_UINT and NC_STRING, only if NETCDF_HAS_NC4 is defined (and  
> only if format=NC4 for writing).
>     Support implemented for variables and attributes.
>     NC_STRING type is supported for reading (scalar and arrays)  
> attributes and is used for writing only for array attributes (scalar are  
> still written as NC_CHAR).
>     If NETCDF_HAS_NC4 is not defined or format!=NC4, NC_STRING array  
> attributes are written as a single NC_CHAR string using the GDAL  
> {v1,v2,...} convention.
>     Add missing support for NC_BYTE in CreateBandMetadata() and  
> NC_BYTE/SHORT in NCDFPut1DVar().
>
> 3) Add support for new NASA Ocean Color L2 products and ESA Sentinel-3  
> L1 or
>     L2 products which use the NetCDF-4 format (with groups for NASA, see
>     http://oceancolor.gsfc.nasa.gov/DOCS/FormatChange.html):
>     - NASA products: simulate a "coordinates" variable attribute to  
> detect CF
>       geolocation arrays, and set bBottomUp to FALSE
>     - ESA products: set bBottomUp to FALSE and disable warning on missing
>       Conventions attribute
>
> 4) Fix bug #4554 with a more generic solution by disabling the  
> installation of the HDF5 atexit() cleanup routine using H5dont_atexit().
>     Previous fix was to call GDALExit() (for the moment only defined  
> gdalwarp.cpp) at the end of every program, which is more painful.
>
> 5) Fix implementation of GetScale/Offset to not always return  
> pbSuccess=TRUE.
>     Fix CopyMetadata to handle bands with only scale or offset.
>     ==> WARNING this commit breaks the autotest netcdf_13 (check for  
> scale/offset = 1.0/0.0 if no scale or offset is available), but for me  
> it is not logical to return always pbSuccess=TRUE
>
> 6) Optimize IReadBlock() and CheckData() handling of partial blocks in  
> the x axis by re-using the GDAL block buffer instead of allocating a new  
> temporary buffer for each block.
>
> 7) Force block size to 1 scanline for bottom-up datasets if nBlockYSize  
> != 1 instead of raising a fatal error
>     ==> Solve a recent problem raised on the mailing list
>
> 8) Implement Get/SetUnitType using the standard "units" NetCDF attribute
>
> 9) Change default block size to 256x256 instead of scanline (only affect  
> file without NetCDF chunking)
>     ==> because I think this is better for a random access to the data,  
> but I'm not sure if the community want this change which could impact  
> performances
>
> 10) I've also implemented for my needs support for 1D variables by  
> simulating 2D datasets with only one row (dimensionless variables are  
> not supported for the moment),
>        but this breaks backward compatibility because files containing  
> only one variable and associated 1D coordinate variables are now seen as  
> multiple sub datasets...
>        and maybe this is not the goal of GDAL to give access to  
> not-2D-raster variables (but sometimes it's useful ;-) )
>
> Thanks for GDAL!
>
> Julien
>
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