Changes between Version 46 and Version 47 of NetCDF_exportUpdateChanges

Timestamp:

Oct 17, 2011, 8:13:12 AM (13 years ago)

Author:

etourigny

Comment:

--

NetCDF_exportUpdateChanges

@@ -124 +124 @@
  * and individual corner coordinates (NN, SS, WE and WW).
 
-The new implementation saves the GeoTransform array only for a geographic CRS, in the specific case that lon/lat values are not written to the file. The corner coordinates are never saved as they are redundant.
-
+The new implementation does not save the GeoTransform array, with the exception of when the GeoTransform cannot be retrieved from the netcdf file (i.e. only for a geographic CRS, in the specific case that lon/lat values are not written ). This has the effect that in some cases, the computed pixel width and height (GT(1) and GT(5)) differs by a very small amount from the original values in the GDAL raster file, due to various floating-point operations and half-pixel shifts. 
+
+For example, 
+{{{ 
+Given a landsat file (tmp/melb-small_AEA.nc) generated by the autotest netcdf_cdf_4:
+
+src:(1232736.8666076646, 30.080459243369152, 0.0, -4118493.9582119514, 0.0, -30.080459243369152)
+dst:(1232736.8666076646, 30.080459243369834, 0.0, -4118493.9582119514, 0.0, -30.080459243366494)
+
+The difference in width/height is less than 10E^-11 (10 pm), while the corners are identical.
+
+However, when translating the original landsat file (melb-small.tif) in UTM, the pixel width/height of 30m is preserved exactly.
+}}}
+
+We feel that this difference is marginal, however should a strong argument be presented, we could save the GeoTransform as before and compare with the values stored in the CF dimensions variable for conflict.
+The corner coordinates are never saved as they are redundant. When importing a CF file, if GeoTransform cannot be computed from dimension variables, GeoTransform and corner coordinates are consulted if available.
+ 
 We propose adding a new WRITE_GDAL_TAGS driver creation option to allow the user to choose not to write these tags. However it would be set to 'YES' by default.
 
-When importing files in NetCDF format, the driver checks the GDAL-written WKT (including Datum) matches the projection information stored in the CF-compliant attributes. If there is a conflict, the CF-compliant CRS will be considered as authoritative.
+When importing files in NetCDF format, the driver checks the GDAL-written WKT (including Datum), matches the projection information stored in the CF-compliant attributes. If there is a conflict, the CF-compliant CRS will be considered as authoritative.
 
 '''Rationale''':