Hi All,
Section 4.1 says:
(http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/cf-conventions.html#coordinate-types)
"/Coordinate types other than latitude, longitude, vertical, and
time _*are allowed*_. To identify generic spatial coordinates we
recommend that the |axis| attribute be attached to these coordinates
and given one of the values |X|, |Y| or |Z|./"
This is in contradiction to section 5.6:
(http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/cf-conventions.html#grid-mappings-and-projections)
"/When the coordinate variables for a horizontal grid are not
longitude and latitude,*_it is required that the true latitude and
longitude coordinates be supplied_* via the coordinates attribute/."
Section 5.6 was added after many years of CF usage in which non-lat-long
coordinate systems were regarded as _valid_ CF files. There are
"climate and forecast" applications in which numerical experiments are
performed in purely theoretical coordinate systems -- say, ocean models
on cyclic coordinates over an idealized ocean basin. (Much less common
today than 20 years ago.)
This contradiction needs to be resolved. My opinion is that it is
section 5.6 that should be revised to say that "grid_mapping" is a tool
that is available **if a mapping is needed** -- rather than as a
required attribute whenever the coordinates are not conventional lat-long.
Opinions?
- Steve
==============================================
On 7/7/2011 4:54 PM, John Caron wrote:
On 6/30/2011 1:26 PM, Randy Horne wrote:
Paragraph 5.6 Coordinate Reference Systems, Grid
Mappings, and Projections, first sentence:
When the coordinate variables for a horizontal grid are not longitude
and latitude, it is required that the true latitude and longitude
coordinates be supplied via the coordinates attribute.
Are there any exceptions to this "rule" ?
The reason for asking is based on a scenario where the coordinate
system and constituent locations for the data points associated with
the gridded data set can be determined solely based on the grid mapping.
The reason for wanting to do this is to minimize the size of the
netCDF file, and the hardware resources required to process, store,
and distribute the file.
Hi Randy:
There has been no formal exception granted from this rule. The java
netcdf library does not require lat/lon as long as a valid grid
mapping is supplied. In that case, the lat/lon arrays are 2D, and can
be a significant size. When writing "CF compliant" netcdf files (eg
from netcdf subset service), the user is allowed to decide to include
lat/lon or not. Technically if not included, the files are not fully
CF compliant.
CF has mostly thought about how to make archival model data as
self-contained as possible, thus the insistence on lat/lon arrays. I
think there are other valid uses of netcdf-CF files, eg as an exchange
format, and that the lat/lon arrays should be optional. There is
currently no mechanism, eg a "profile" of CF to allow that.
John
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