Changes between Version 47 and Version 48 of NetCDF_exportUpdateChanges


Ignore:
Timestamp:
Oct 17, 2011, 9:26:47 PM (13 years ago)
Author:
pds
Comment:

Adding notes about projection parameters - 1.

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  • NetCDF_exportUpdateChanges

    v47 v48  
    8080
    8181The proposed changes in this section update the NetCDF driver when exporting to NetCDF to:
     82 * Ensure the projection parameters are saved correctly according to their CF-1 definitions
    8283 * save extra information so datasets are CF-1 compliant and can be viewed as gridded data in CF-1 compliant NetCDF applications built on NetCDF Java (X/Y projected coordinate arrays, and optionally full !Lat/Lon arrays);
    8384 * save as much of the SRS information in CF-1 compliant format as possible (including ellipsoid parameters);
    … …  
    8788The following subsections describe these changes specifically.
    8889
    89 ==== 3.1) When exporting to NetCDF map projections via GDAL, save projection coordinate variables. Optionally, also save lat and lon mapping variables. ====
     90==== 3.1) Ensure projection parameters are saved and read from correctly for all projections specified in CF-1.5 ====
     91
     92As logged at #2893 and #3324, the NetCDF driver as it stood in 1.8 was not able to write CF-1.5 compliant attributes for projections such as Lambert Conformal Conic.
     93
     94A thorough analysis of the CF-1.5 map projections supported (see again [[http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/apf.html CF-1.5 App F]]) showed that the mapping of OGC WKT projection parameter names to CF-1.5 equivalents needed to be done per-projection. Whereas, the existing code tried to use a single global mapping of WKT attribute names to CF-1 for use in exporting.
     95
     96To overcome this issue we have defined per-projection mappings of OGC WKT parameter names to CF-1.5 equivalents using simple data structures, and re-written the NetCDF driver export code (in the CreateCopy() function) to reference these mappings when saving a map-projected raster.
     97
     98This was tested for all projections listed as part of CF-1.5 by translating input files in another format (eg GeoTIFF) into NetCDF, and then ensuring the resulting files were CF-compliant and could be opened as gridded data by the NetCDF Java API.
     99
     100We also tested these files could have their projections correctly re-imported back into GDAL :- and in the process updated the NetCDF driver's projection import code (in !SetProjection()) as well.
     101
     102
     103
     104==== 3.2) When exporting to NetCDF map projections via GDAL, save projection coordinate variables. Optionally, also save lat and lon mapping variables. ====
    90105
    91106'''Related fix''': #2893 [[BR]]
    … …  
    112127Thus we've chosen to make it ''optional'' to write these Variables - but provide the 'WRITE_LONLAT' co option for users who wish to make sure their data is fully self-describing and CF-1 compliant, without relying on on-the-fly reprojection.
    113128
    114 ==== 3.2) Continue saving GDAL custom attribute tags in addition to CRS definition, but add option not to save ====
     129==== 3.3) Continue saving GDAL custom attribute tags in addition to CRS definition, but add option not to save ====
    115130
    116131'''New -co options''': -co WRITE_GDAL_TAGS=YES/NO (default: YES)[[BR]]
    … …  
    153168The GeoTranform array is not written to file unless there is no means to recover that information (i.e. in the case when lon/lat values are not written to file for a geographic CRS). 
    154169
    155 ==== 3.3) Add saving of reference Ellipsoid parameters in CF-1 compliant format used by dataset ====
     170==== 3.4) Add saving of reference Ellipsoid parameters in CF-1 compliant format used by dataset ====
    156171
    157172'''New -co options''': No, though related to the new proposed WRITE_GDAL_TAGS option below.
    … …  
    206221   * Code and history of changes is in a git repository at https://github.com/etiennesky/gdal-netcdf/ (gdal-netcdf-proj branch)
    207222   * Since the modifications above involved a lot of related changes to significant parts of the driver, we managed the actual development process using a Github repository for integration and testing.   
    208    * The autotest suite had been modified (in svn trunk) to test the improvements
     223   * The autotest suite had been modified (in svn trunk) to test the improvements - including using an online cf-checker to test conformance when available.
     224 * We also manually tested that  NetCDF files created by the updated driver in each supported CF-1.5 projection could be opened via NetCDF-Java, in addition to the cf-checker used in the new autotests. This has allowed verification of the majority of projections, with a few remaining issues - see link below.
    209225 * We will ask the users of the driver to please test the new version with their important files and notify any regressions.
    210226
    … …  
    213229 * The code will be committed to svn trunk, and if possible will be back-ported to the 1.8 series (should a 1.8.2 release occur after implementation and sufficient testing).
    214230 * The commit will happen as soon as we get approval from members of the gdal-dev list or RFC vote (if needed)
     231
     232== Appendices/Links ==
     233
     234TODO - links to Wiki page with status of projection-handling by driver in development. Will be useful to point people to this, if interested to help with testing.