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 <[email protected]> ----- > Date: Tue, 8 Oct 2013 11:20:18 -0400 > From: Nan Galbraith <[email protected]> > User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:17.0) > Gecko/20130801 Thunderbird/17.0.8 > To: [email protected] > 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 [[email protected]] > >>>Sent: 30 May 2013 15:22 > >>>To: Lowry, Roy K. > >>>Cc: [email protected]; [email protected] > >>>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." <[email protected]> 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 [[email protected]] On Behalf Of > >>>>Matthias Lankhorst [[email protected]] Sent: 30 May 2013 13:16 > >>>>To: [email protected] > >>>>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 * > ******************************************************* > > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
