Update NetCDF driver for greater CF-1 convention compatibility (orientation, datums, map projections)

(Note: this is an initial draft by pds - to collect related ideas for input and if necessary RFC development)

Status: Development

Summary

In August 2011 Etienne Tourinsky initiated a discussion on the GDAL mailing list proposing and calling for interest in improvements to GDAL's NetCDF driver (Subject: discussion on improvements to the NetCDF driver and CF-1 convention), and collected ideas at wiki:NetCDF_Improvements.

This page summarises several of the proposed improvements that would improve the compatibility of GDAL's NetCDF driver with the CF-1 NetCDF conventions :- but change the default behaviour of the driver and thus affect backward compatibility.

The changes affect orientation of data, handling of datum parameters, writing coordinate variables, and are described specifically below.

Rationale

The common rationale underlying these changes is:

(1) The NetCDF view of rasters created by GDAL should conform to CF-1 conventions :- and work for key operations with common NetCDF tools such as creating a WMS or WCS display from a NetCDF raster (eg displaying a WMS using ncWMS, or a WCS using Thredds Data Server).

• If the user wishes to go against NetCDF conventions when exporting, they should use a -co option to specifically request this.

(2) Given (1), the driver should use the CF-1 conventions for storing needed metadata where at all possible, but where necessary add extra metadata so the file can be readable back into GDAL with no loss of information or translation error. A specific example of this is named Datum and EPSG codes, which GDAL needs to supplement CF-1.

• Again, if the user doesn't wish to supplement the files with GDAL metadata, they should specifically disable this using the appropriate -co option.

(3) Given (2), when importing from NetCDF files, the driver will first read from CF-1 conventions, then read supplementary data from GDAL-added attributes. If the two sources conflict, we should report this and consider the CF-1 encoded info as authoritative, which assumes a user has modified the data with a non-GDAL program.

Planned Changes

Change to a default south-up orientation for NetCDF data

Related tix: #4251

New -co options: ?? Should we include an option to allow force assume south-regular?

Description:

When GDAL exports to NetCDF, translate the data so that it is in "south up" orientation, i.e. invert the data from the GDAL default. Likewise upon import from CF-1, assume the data is in "south up" orientation, and translate the y values by default.

Rationale:

CF-1's default orientation is south-up, and all known current CF-1 -compliant applications expect this.

Continue saving NetCDF custom attribute tags by default, but add option not to save

Related tix: N/A
New -co options: -co WRITEGDALTAGS (yes/no, defaults to yes)
Has back-compat impact?: No

Change:

Currently, as well as following the CF-1 conventions to save projection metadata, the NetCDF driver also writes metadata in several custom attributes:

• A "spatial_ref" tag containing full OGC WKT string, including datum and EPSG codes if known
• A geotransform array, saved as a string (GeoTransform).

The WRITEGDALTAGS driver creation option would allow the user to choose not to write these tags. However it would be set to 'YES' by default.

Rationale:

We assume by default that if users export to a NetCDF file from GDAL, then they may wish to import into GDAL again and retain full information. Currently, the CF-1 convention doesn't allow saving all the projection information that GDAL can manipulate via a WKT (such as named Datums, and EPSG authorities).

However, there is certainly a use-case to export to a NetCDF file with no intention to access again via GDAL and use purely for use by CF-1 applications, in which case we don't need to save the added metadata.

When exporting to NetCDF map projections via GDAL, save projection coordinate variables. Optionally, also save lat and lon mapping variables.

*Related tix*: #2893
*New -co options*:

*Rationale*: X and Y needed as part of CF-1 conventions - and without them, files are not recognised as gridded data by the NetCDF-Java API, and thus ncWMS, ToolsUI and TDS.

They do not add a significant file-size overhead.

Similarly,

Related issues:

• South-up change on both import and export?
• adding X and Y coords

Compatibility Issues

Test Suite

Documentation

Addenda

• We also intend to improve interface for writing to NetCDF 4 - (but will handle this separately, may not need an RFC)