Dear all,
another view:
Can't remember *all* the issues here, but certainly reporting the
latitude and longitude points for GCM grids without further precision
(e.g., information on the figure of the Earth) is sufficient for any
comparison with observations. Only certain (usually prescribed)
conditions at the earth's surface (e.g., surface height) coming from a
GCM should be trusted at the individual grid point scale, and no
sub-grid scale information is directly available from the GCM
(normally). So, even if a station data is near the boundary of a GCM's
grid-cell, it should hardly matter which of the grid cells it straddles
you compare it to. The GCM sort of gives you a grid cell average value
that applies to some region in the vicinity of the cell. So, it
doesn't matter where you think it is precisely located.
Down-scaled output from the GCM will be at higher resolution, but again
since the original data doesn't apply at a point but for a general
region (usually quite a bit larger than 12 km, and even if it weren't we
wouldn't believe stuff going on at that scale), so where the cell is
exactly located again doesn't matter.
best regards,
Karl
On 7/27/11 4:38 AM, David Blodgett wrote:
Without the grid_mapping, the lat and lon still make sense in the
common case
(and original CF case) of GCM data, and in many other cases, the intended
usage of the data does not require precision about the figure of the
Earth.
Although this metadata could be valuable if it can be defined, I
think it would
be too onerous to require it.
I hope to present on this very issue at AGU. The problem we see with
ambiguous definition of datums is a cascade of non-recognition of
datums through processing algorithms and in the output of some
processes that generate very detailed data.
The prime example is downscaled climate data. Because the climate
modelers involved generally consider lat/lon to be a lowest common
denominator, the datum used to geolocate historical data (like rain
gages) is neglected. What results is, in our case, a 1/8deg (12km)
grid with no datum. This is unacceptable. As at this resolution, the
errors in a wrong assumption of datum for the grid can cause very
substantial (a full grid cell or more) geolocation errors.
If the CF community intends to consume any ground based data, then
datums must be preserved from ingest of ground based forcing
throughout data storage and processing. This is fundamental
information that is required for ALL data comparison operations.
I would argue that CF compliance should require this information. This
puts the requirement to make metadata assumptions on data
publishers/producers rather than data consumers. It is unacceptable to
have different data consumers making different assumptions of
geolocation on the same data.
Off soapbox.
Dave Blodgett
Center for Integrated Data Analytics (CIDA)
USGS WI Water Science Center
8505 Research Way Middleton WI 53562
608-821-3899 | 608-628-5855 (cell)
http://cida.usgs.gov <http://cida.usgs.gov/>
On Jul 26, 2011, at 5:24 AM, Jonathan Gregory wrote:
Dear all
For datasets which are intended for analysis by end-users I think it
would be
undesirable to remove the requirement of providing explicit lat and lon
coords even if a grid_mapping is provided. I think it is unrealistic
to expect
all software which someone might use to analyse netCDF files to be
able to
recognise and act upon all possible values of the CF grid_mapping
attribute,
and without the lat and lon information the user would have a
problem. If the
issue is storage space in the file I think the much better choice is
to store
the explicit coordinates in another file, by extending the CF
convention to
allow datasets to be distributed over several linked files, as
gridspec does
for example.
Steve appears to suggest that grid_mapping is required in some
circumstances,
but I don't think it is at present. However, the text Steve quotes
may not be
quite right:
"/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/."
The text should make it clear that this requirement applies when the
data has a
geolocated horizontal grid. It doesn't necessarily apply to idealised
cases.
We could clarify this with a defect ticket.
Cheers
Jonathan
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