Dear Nan
"sum" is the correct cell_methods to indicate that a quantity applies to the
whole extent of a cell i.e. it is extensive, rather than intensive. It does
not necessarily mean that values were actually added up. For instance, "sum"
would be the cell_methods for a precipitation amount accumulated within a
time-interval.
Best wishes
Jonathan
----- Forwarded message from Nan Galbraith <ngalbraith at whoi.edu> -----
> Date: Tue, 8 Oct 2013 11:20:18 -0400
> From: Nan Galbraith <ngalbraith at whoi.edu>
> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:17.0)
> Gecko/20130801 Thunderbird/17.0.8
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] standard_name for acoustic travel time from echo
> sounder
>
> Hi All -
>
> Can someone please answer this question? There must be analogies in
> the atmospheric community, but I don't know of anyone putting this kind
> of ocean data into CF.
>
> What is the best way to provide the information needed to extract the
> geophysical quantity sound speed from the measured quantity of signal
> travel time?
>
> It has been proposed to be done with cell_methods='Z:sum', or with
> a coordinate reference frame that includes the orientation of the
> instrument.
> Is there a 'best practice' for this?
>
> Thanks - Nan
>
>
>
>
> On 6/7/13 3:57 PM, Nan Galbraith wrote:
> >Hi Matthias and all -
> >
> >Are cell methods the right way to document this? The 'sum' cell
> >method indicates
> >that you've summed a number of measurements, and I don't think
> >that's the case
> >here.
> >
> >I'd have thought that providing the the instrument depth and
> >orientation (upward)
> >would make it more clear.
> >
> >This isn't a feature I use routinely, so I could easily be missing
> >something.
> >
> >Cheers - Nan
> >
> >On 5/30/13 6:12 PM, Matthias Lankhorst wrote:
> >>Hi,
> >>
> >>thanks already for these comments. Roy's suggested name sounds
> >>pretty to my
> >>ears.
> >>
> >>Trying to explain my "cell_methods" thing:
> >>
> >>The instrument is sitting at a fixed spot on the seafloor, so
> >>unlike the echo
> >>sounder on a ship, the distance does not change (well, there are
> >>tides, but we
> >>filter them out). The remaining signal variance is variability in
> >>environmental sound speed, which is mostly a measure of sea
> >>water temperature.
> >>
> >>The data, although measured by an instrument at one spot, are
> >>dependent on the
> >>vertical distance that the acoustic signal travels, i.e.
> >>represent some space
> >>other than a single point. Chapter 7 of the CF document that I
> >>found online
> >>explains it this way: "When gridded data does not represent the
> >>point values
> >>of a field but instead represents some characteristic of the
> >>field within
> >>cells of finite "volume," a complete description of the variable should
> >>include metadata that describes the domain or extent of each cell..."
> >>
> >>In my example, let us assume my IES sits at 4500m depth looking up. The
> >>acoustic signal travel time (roundtrip) will be about 6 seconds
> >>(sound speed
> >>is ca. 1500 m/s). My data will be numbers that are closer to 5.9
> >>seconds if it
> >>is warm (faster sound speed), and more like 6.1 seconds if it is cold.
> >>
> >>If my instrument were instead sitting in the same body of water
> >>at 3000m depth
> >>(let's assume there is a mountain nearby), all of my numbers would be
> >>something close to 4 seconds. Now... I don't want the user to
> >>think I am still
> >>in 4500m depth in outrageously hot water!
> >>
> >>Bottom line: I need to tell the user what depth range I am
> >>covering (0-4500 or
> >>0-3000), and in my limited understanding of the situation this
> >>is done via the
> >>cell_methods and cell_bounds attributes.
> >>
> >>Best wishes, Matthias
> >>
> >>
> >>On Thursday, May 30, 2013 09:49:29 am Lowry, Roy K. wrote:
> >>>Hi John,
> >>>
> >>>Not exactly. The travel time in both water column echosounding and
> >>>seismics is a proxy for distance and therefore provides
> >>>information on the
> >>>vertical distribution of returned signal intensity.
> >>>
> >>>Cheers, Roy.
> >>>
> >>>________________________________________
> >>>From: John Graybeal [graybeal at marinemetadata.org]
> >>>Sent: 30 May 2013 15:22
> >>>To: Lowry, Roy K.
> >>>Cc: mlankhorst at ucsd.edu; cf-metadata at cgd.ucar.edu
> >>>Subject: Re: [CF-metadata] standard_name for acoustic travel
> >>>time from echo
> >>> sounder
> >>>
> >>>+1 for Roy's choice.
> >>>
> >>>Can you explain the following for the acoustically naive? "I assume the
> >>>data would need some additional description to denote the
> >>>vertical extent
> >>>of the measurement, such as cell_bounds and cell_methods='Z:sum'."
> >>>
> >>>John
> >>>
> >>>On May 30, 2013, at 06:45, "Lowry, Roy K." <rkl at bodc.ac.uk> wrote:
> >>>>Dear All,
> >>>>
> >>>>Of Matthias's suggestions I have a strong preference for a slight
> >>>>extension of roundtrip_acoustic_travel_time_in_sea_water, namely
> >>>>acoustic_signal_roundtrip_travel_time_in_sea_water. 'two-way' is a
> >>>>possible alternative to 'roundtrip' but I think the former carries
> >>>>unfortunate seismic semantic implications, so 'roundtrip' is
> >>>>better for
> >>>>IES. Including 'in_sea_water' is also essential to clearly
> >>>>distinguish
> >>>>it from seismic data.
> >>>>
> >>>>Cheers, Roy.
> >>>>________________________________________
> >>>>From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] On Behalf Of
> >>>>Matthias Lankhorst [mlankhorst at ucsd.edu] Sent: 30 May 2013 13:16
> >>>>To: cf-metadata at cgd.ucar.edu
> >>>>Subject: [CF-metadata] standard_name for acoustic travel
> >>>>time from echo
> >>>>sounder
> >>>>
> >>>>Dear CF,
> >>>>
> >>>>I have oceanographic data from IES instruments (inverted echo sounder)
> >>>>that I would like to publish via OceanSITES in a
> >>>>CF-compliant form. The
> >>>>data in question are acoustic travel times from the echo sounding
> >>>>device. This means the time it took for the acoustic signal
> >>>>to run from
> >>>>the instrument (which sits on the seafloor) to the sea
> >>>>surface and back
> >>>>to the instrument. These data are commonly used as a proxy for ocean
> >>>>heat content.
> >>>>
> >>>>I don't think there is a suitable CF standard_name out
> >>>>there, and ask for
> >>>>your help in finding/creating one. Which of the following sound good?
> >>>>
> >>>>acoustic_travel_time
> >>>>vertical_acoustic_travel_time
> >>>>roundtrip_acoustic_travel_time_in_sea_water
> >>>>echo_sounder_acoustic_travel_time
> >>>>
> >>>>...I could think of a couple more combinations using the
> >>>>words above, but
> >>>>would like to hear other people's opinions.
> >>>>
> >>>>The canonical units would obviously be seconds.
> >>>>
> >>>>I assume the data would need some additional description to denote the
> >>>>vertical extent of the measurement, such as cell_bounds and
> >>>>cell_methods='Z:sum'.
> >>>>
> >>>>Any comments?
> >>>>
> >>>>Kind regards, Matthias
> >>>>
> >>>>
> >
> >
>
>
> --
> *******************************************************
> * Nan Galbraith Information Systems Specialist *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 (508) 289-2444 *
> *******************************************************
>
>
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----- End forwarded message -----
Received on Tue Oct 08 2013 - 10:15:56 BST
