Hi Alison,
*About the 'water level' vs 'sea surface height'
In the case of Jeff needs would only concern ocean data, would you mind if I ask CF to introduce both? I was about to ask CF for such quantities on rivers and lakes..
*About reference datum, my opinion is that considering CF standard name is supposed to uniquely identify one geophysical field, we should ask: "are there as many standard names as there are datums?"
Is the sea surface height above French Naval Chart Datum geophysically different from the sea surface height above French Naval Chart Datum for Lebanon? If the answer is yes, I guess introducing a new datum attribute in CF would be relevant.
By the way if we consider the sea surface height above geoid, geoid is not exactly a datum, or became indirectly a datum: the whole quantity refers first to the geophysical information retrieved, which is the absolute ocean topography, and which is not obtained by measuring some heights and subtracting geoid heights from. It is really full of meaning in terms of ocean dynamics - and less on the way it was really measured..
*I confirm that what you explained about distinction between the same geophysical field obtained by different sensors was also applied by OSTST/Jason community. In the NetCDF product you can have 2 variables with the same standard name, but one is obtained via the onboard instrument, and the second via ECMWF forecasts. It is another illustration.
Cheers,
Olivier.
-----Message d'origine-----
De : cf-metadata-bounces at cgd.ucar.edu [mailto:cf-metadata-bounces at cgd.ucar.edu] De la part de alison.pamment at stfc.ac.uk
Envoy? : vendredi 12 f?vrier 2010 12:15
? : cf-metadata at cgd.ucar.edu
Objet : Re: [CF-metadata] seeking CF name for total water column height
Dear Jeff,
Thank you for your standard name proposal for water column height and
thanks to all who have contributed comments to this discussion. The
consensus view seems to be that
> sea_floor_depth_below_sea_surface
is an acceptable name for this quantity and that it is consistent with
the existing standard name sea_floor_depth_below_sea_level. The
canonical units will be metres (m). Roy has suggested the following
definition:
> 'The vertical distance between the sea surface and the seabed as
measured at a given point
> in space including the variance caused by tides and possibly waves.'
If no further comments on this name or its definition are received over
the next seven days then they will be accepted in their present form for
inclusion in the standard name table.
You have also suggested two further standard names:
water_level_with|above_reference_datum
water_level_without_reference_datum
For the latter name, Roy has suggested the term 'tide gauge zero' to
express the lack of reference datum.
Am I correct in thinking that all these quantities are referring to
measurements in the open sea or coastal areas? If so, I think we should
refer to sea_surface_height rather than water_level for consistency with
other names. So we could have:
sea_surface_height_above_reference_datum
sea_surface_height_above_tide_gauge_zero.
I am wondering how many different reference datums there are likely to
be? You mentioned low water and highest astronomical tide but are there
likely to be dozens of these quantities? If there are only a few we
could consider introducing separate standard names for them as we have
done with quantities such as
sea_surface_height_above_reference_ellipsoid and
sea_surface_height_above_geoid for example. If there are a large number
of possible datums then it wouldn't be practical to introduce standard
names for them all and we would need another way to record which datum
is being used. Regarding the definitions, do both quantities average out
the effects of waves?
Nan has raised the question of how to distinguish between the same
geophysical quantity measured with different sensors/sensor
configurations or post-processed differently. This question has arisen
in a number of contexts recently. For example, as well as sea water
pressure and depth measurements, we discussed some CMIP5 proposals in
which climate models were simulating cloud amounts as retrieved from two
different satellite instruments. In the case of CMIP5 I believe the
issue was resolved by using the same standard name for both quantities
and placing them in differently named files, but a simple way of
distinguishing between instruments/instrument types/retrieval algorithms
in metadata would be to use the 'source' attribute which can be either
global or attached to a single variable (this is already part of the CF
conventions, section 2.6.2).
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Fax: +44 1235 446314
Rutherford Appleton Laboratory Email: alison.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.
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