Roy Lowry wrote:
> 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.
>
It does seem like we have departed rather far from the original question
of differentiating between fixed and tethered sensors.
I think we are veering way too far towards embedding instrument metadata
(and even independent coordinate information) into the standard name.
If the quantity in question is sea water pressure then it should be
called "sea_water_pressure". If it is measured at 200m then it should
have a typical CF-encoded Z coordinate axis to indicate that it is at
200m depth. In fact wherever applicable the typical CF coordinate axes
should be used, so that the geopositioning is encoded in an
interoperable way. Embedding that semantics into the name is far less
desirable. If the instrument is on the sea floor, then the depth of the
sea floor should be used for its Z axis coordinate. How to encode the
additional bit of semantics to indicate that this depth is in fact the
sea floor is also important, but it should be raised as a separate
question. Knowing the position is on the sea floor is equally
applicable to pressures, temperatures, currents, etc. -- it shouldn't be
part of the standard name for a pressure in particular.
Regarding the question of tethered sensors, my initial reaction was that
this should not be encoded in the standard name. The physical quantity
is still simply sea water pressure. What is special about the
observation type is that the vertical coordinate is a function of time
-- a 1D trajectory if you will. (Actually the cable movement is a 3D
trajectory, but if the data provider wants to view this as a point in
X,Y who am I to quibble?) I was unclear, however, whether the data
providers would always have all of the information needed to encode the
vertical position as a function of time .... Lacking that information,
maybe a new standard name is unavoidable.
- Steve
> 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
>> _______________________________________________
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>> CF-metadata at cgd.ucar.edu
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>>
>
>
> 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
>
> _______________________________________________
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>
>
>
--
Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
7600 Sand Point Way NE, Seattle, WA 98115-0070
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"The only thing necessary for the triumph of evil is for good men
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