Changes between Version 6 and Version 7 of NetCDF_exportUpdateChanges

Timestamp:

Sep 21, 2011, 10:30:37 PM (13 years ago)

Author:

pds

Comment:

Wrote notes on lat and lon arrays.

NetCDF_exportUpdateChanges

@@ -65 +65 @@
 === When exporting to NetCDF map projections via GDAL, save projection coordinate variables. Optionally, also save lat and lon mapping variables. ===
 
-*Related tix*: #2893 [[BR]]
-*New -co options*: 
+''Related tix'': #2893 [[BR]]
+''New -co options'': 
+  * WRITELONLAT=yes/no (default: yes for geographic, no for projected)
+  * TYPELONLAT=float/double (default: double for geographic, float for projected)
 
-*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, 
+''Has back-compat impact?'': No
 
-Related issues:
- * South-up change on both import and export?
- * adding X and Y coords
+''Change'':
 
+When exporting rasters to NetCDF, save Coordinate Variables for the projected SRS, as required by the CF-1 convention, normally saved as "x" and "y".
+
+Optionally using -co options listed above, also write 2D 'lat' and 'lon' arrays, which are specified as part of the CF-1 convention for map projections. 
+
+''Rationale'':
+
+The CF-1 conventions specify these coordinate variables as necessary for grids, see [[http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/ch05s06.html ch5.6]] and [[http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/apf.html App F]] in the document. 
+
+Tests with NetCDF CF-1 reading software show that rasters with projected grids will not be correctly interpreted without the coordinate variables in the projected system (the NetCDF-Java API determines this, since it's used by ncWMS, ToolsUI and Thredds Data Server).
+
+While the CF-1 convention examples also contain 2D "lat" and "lon" Variables containing the latitude and longitude at each data point, tests have shown that the NetCDF Java API can correctly interpret CF-1 data without these added. Thus we've chosen to make it ''optional'' to write these Variables, as they are redundant from GDAL's point of view (Lat and Lon are calculated on-the-fly), and they increase the size of a NetCDF file significantly. (E.g. if the actual band-data is stored as a 2D Byte array, but the lat and lon are stored as doubles, then the file-size can easily increase by 10X).
 == Compatibility Issues ==