Support Create() method for NetCDF driver so that gdalwarp can be used on a netcdf file

[pds]

We'd like to be able to use gdalwarp functionality with NetCDF files as an output, which requires supporting the Create() and IWriteBlock() methods for the NetCDF driver.

The NetCDF C API supports creating a file with empty variables, then later updating an existing file, and random-access writing, all prerequisites for the Create() method in GDAL.

This should likely involve refactoring the GDAL driver so common functionality is shared between the Create() and CreateCopy() methods.

[etourigny]

Re-factored the driver and added a Create() function, commited to trunk (r23456), and autotests (r23457).

Significant changes brought by this commit:

• removed vars that are used temporarily (e.g. panBandZLev)
• Set* functions write to netcdf file when in update mode
• Create() and CreateCopy() common functionality in CreateLL()
• write bands now in IWriteBlock()
• bottom-up = FALSE by default
• updated all sprintf to (%.8g / %.16g) because of projection parameter precision differences

[etourigny]

committed small fixes to trunk (r23458, r23459, r23460)

[Even Rouault]

Etienne, you might want to check the correctness of the first part of the following fix :

r23461 /trunk/gdal/frmts/netcdf/netcdfdataset.cpp: Fix compiler warnings about wrong enum comparison, and uninitialized variable

[etourigny]

Fix is fine, thanks. It worked before by chance, because NC_BYTE and GDT_Byte have the same value.

r23463: Added autotest for Create() and CreateCopy() a band with unsigned byte data and fix to PIXELTYPE metadata when calling Create().

[etourigny]

In GDAL Byte data is unsigned by default and uses PIXELTYPE=SIGNEDBYTE metadata to specify signed byte, following

http://trac.osgeo.org/gdal/wiki/rfc14_imagestructure

Byte data in netcdf is by default signed, and the following conventions are used by the driver to specify unsigned byte

• valid_range: ​http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/Attribute-Conventions.html
• _Unsigned : ​http://www.unidata.ucar.edu/software/netcdf/docs/BestPractices.html
• NC_UBYTE for NC4 format

[etourigny]

Using gdalwarp on a large dataset with -co COMPRESS=DEFLATE option (which forces NC4C format) does not work (takes a very long time). This is probably due to wrong chunksize, which was optimized for CreateCopy(). Exporting without compression works, so use that for now.

[etourigny]

r23635 : properly enable chunking in netCDFDataset::DefVarDeflate() (#4404 and #4221)

[etourigny]

closing this ticket, see related bugs #4404 and #4432