Hello Jonathan,
My (and I'm pretty sure John's) concern is how do we stop someone after bathymetric depths taking a channel labelled 'sea_water_pressure_at_sea_floor' converting to depth and averaging to get a point bathymetric depth relative to mean sea level when the measuring instrument is 200m above the seabed thus giving them a value wrong by 200m?
If you can give me a clear explanation of how 'sea_water_pressure_200m_above_the_seabed' may be unambiguously described in CF then I will cease to be concerned. However, we still haven't solved Nan's problem of differentiating between fixed and tethered sensors 200m above the seabed. This is important because the former is a reliable measure of sea level but the latter isn't.
Cheers, Roy.
>>> John Graybeal <graybeal at mbari.org> 08/28/08 5:12 AM >>>
I can't reconcile Jonathan's answer (sea_water_pressure_at_sea_floor)
with Nan's case 2 ("mounted rigidly at a fixed location at *or above*
the seafloor"). I would agree that sea_water_pressure is accurate, as
far as it goes -- the meaning of the value is the same, it's the
application of that meaning that is different. (Depending on your
definition of 'meaning', of course.)
There would seem to be two key points about the sensor mounting, one
that it is mounted at a fixed location, and the other is where that
mounting is with respect to multiple axes. (I mean, for the data to
be usefully interpreted, it isn't enough to know just distance above
sea floor, you need to know exactly which lat/lon | X/Y location it is
above, or the height of the sea floor at that location; ideally you'd
know both. In the second case it would be acceptable to have a
reference with respect to some other datum, for example relative to an
altitude from GPS when the sensor is a known distance down from that
measurement, as on a pier). Understood that the request was a deep
ocean context but it would be nice of the solution worked in all
applications.
My bias is always to have the standard name focus on substance +
property, and for other metadata to explain the rest. So I liked the
approach that separated the mounting information from the standard
name. While the argument can be made that the two mountings are not
comparable, and so different names should be used, a legitimate
counter-argument is that with certain processing, the data from the
two mountings could be made comparable, at least in some circumstances.
John
On Aug 27, 2008, at 12:22 PM, Jonathan Gregory wrote:
> Dear Steve, Nan et al.
>
> I think it would be fine to have a CF standard name of
> sea_water_pressure_at_sea_floor
> which is certainly an oceanographically useful quantity. We haven't
> used the
> phrase at_sea_floor hitherto, but sea_floor appears in other
> standard names
> and there are many quantities which have at_LEVEL phrases.
>
> The pressure measured at a depth specified by a vertical coordinate
> would
> naturally be called just sea_water_pressure, I think. This is also an
> oceanographically meaningful quantity in its own right. The purpose
> for which
> you use it (deducing the tilt of the sensor) does not have to be
> indicated by
> the standard name.
>
> Best wishes
>
> Jonathan
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
On Aug 19, 2008, at 7:42 AM, Nan Galbraith wrote:
> 2) Bottom pressure sensors, e.g. mounted rigidly at a fixed
> location at or above the seafloor, where the pressure values
> are a dynamically important quantity that integrates water
> column weight (cf. air pressure on weather charts).
--------------
John Graybeal <mailto:graybeal at mbari.org> -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Interoperability Project:
http://marinemetadata.org
_______________________________________________
CF-metadata mailing list
CF-metadata at cgd.ucar.edu
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
--
This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.
Received on Thu Aug 28 2008 - 03:13:58 BST
