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Ben, Ethan's example has shown clearly the need of such two-time grid in WCS so that it's a good idea to have a use case document and to this capability included, as an extension I suppose, in the next version. BTW, for the irregular grid point case (i.e., irregular pressure levels) we discussed in the last telecon, there might be additional complications by simply allowing omission of offset value(s). This is because WCS is supposed to deliver customized data (e.g., at specific CRS, resolution, number of dimensions and dimension sizes, and format) but a default offset (i.e., omission offset in the request) may result in an unknown number of grid points along a dimension. I hope that you may have found time discussing these domain related issues during the TC meeting. Regards, Wenli ________________________________ From: wcs-1.2.swg-bounces+wenli.yang-1=nasa.gov@xxxxxxxxxxxxxxxxxxxxxxxx [mailto:wcs-1.2.swg-bounces+wenli.yang-1=nasa.gov@xxxxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Ben Domenico Sent: Friday, March 27, 2009 9:23 PM To: Ethan Davis Cc: wcs-1.2.swg@xxxxxxxxxxxxxxxxxxxxxxxx; Unidata GALEON Subject: Re: [WCS-1.2.swg] Forecast times and dimensionality (domain axes) Hi Ethan and Wenli, When we get a chance, it would be good to turn your valuable email exchange into a use case document for having two time dimensions. This is really a practical description of how datasets with two time dimensions are now being served to the community. I hope someone will help me remember this after the OGC TC meetings.' Thanks. -- Ben On Fri, Mar 27, 2009 at 4:02 PM, Ethan Davis <edavis@xxxxxxxxxxxxxxxx<mailto:edavis@xxxxxxxxxxxxxxxx>> wrote: Hi Wenli, Yang, Wenli (GSFC-610.2)[GEORGE MASON UNIVERSITY] wrote: > Ethan, > > John's poster is very nice. As to the 5D cubes, it looks like > they are conceptual rather than true 5D data arrays. If both > t1 and t2 are treated as independent dimensions in a 5D data > array, you'll have no-value cells at the bellow the diagonal > line in the (t1,t2) plane unless you include hindcast/nowcast > results in the data model. The poster actually shows that no > data are available at the lower left part of the 2D (i.e., > model run versus forecast time) plane. They are somewhat sparse arrays but they are not just conceptual. Here is an OPeNDAP data descriptor URL for one of the 5D cubes: http://motherlode.ucar.edu:8080/thredds/dodsC/fmrc/NCEP/GFS/Alaska_191km/NCEP-GFS-Alaska_191km_fmrc.ncd.dds All of of the data variables (coverage fields) have two time dimensions, "run" and "time" (or "time1"). Many, like Temperature, are 5D cubes though some, like Pressure, don't have a vertical dimension Temperature[run = 243][time = 21][isobaric1 = 9][y = 39][x = 45] Pressure[run = 243][time = 21][y = 39][x = 45] Because the arrays are somewhat sparse, a particular subset of the data may return lots of "missing values" (or NaNs). Note: We don't support WCS requests on the 5D cube but most (all?) of the slices do. > I also obtained a netCDF file using the netcdfsubset link, > with u_wind be subsetted. The file contains a 3D (x,y,t) > array instead of 4D (x,y,t1,t2) array. One of the time > dimension, model run, is apparently indicated by the file > name, i.e., NCEP-GFS-Alaska_191km_RUN_2009-03-25T12_00_00Z.nc. That is right, the data you got was from the set (one for each model run) of "Forecast Model Run" 4D slice datasets. This one for the 12Z run on the 25th. If you go up one level in the catalogs to http://motherlode.ucar.edu:8080/thredds/catalog/fmrc/NCEP/GFS/Alaska_191km/catalog.html and follow the "Forecast Model Run Collection (2D time coordinates)" link you will get to the full 5D cube. The next four links all lead to a single slice ("Best Time Series") or a collection of slices ("Forecast Model Run", "Constant Forecast Offset", and "Constant Forecast Date") through the 5D cube. The last link leads to the collection of underlying data files ("File Access"). > In WCS, if you define a grid having a 5D CRS (x,y,z,t1,t2), > with dimension sizes being Nx,Ny,Nx,Nt1,Nt2. A client expects > the returned grid to have Nx * Ny * Nx * Nt1 * Nt2 gird point > values, assuming no subsetting and resampling, unless the server > outputs multiple files (or one file with multiple variables) > each representing one specific time for one of the time dimension > (say, t1) and containing a 4D data array (say, x,y,z,t2). In > this case, the server can avoid no-value points if the sizes of > the t2 dimension in different arrays are allowed to be different, > e.g., the dimension size in the first array being Nt2, in the 2nd > array being Nt2-1, in the 3rd array being Nt2-2, etc. That sounds correct. Though in that case, how would the server describe how the individual files in the response fit together. The client would have to do some work both to 1) understand that they aren't getting back all the data they requested and 2) understand how the pieces fit together and then fit them back together. Ethan > Wenli > > ________________________________________ > From: Ethan Davis [edavis@xxxxxxxxxxxxxxxx<mailto:edavis@xxxxxxxxxxxxxxxx>] > Sent: Wednesday, March 25, 2009 11:35 AM > To: Yang, Wenli (GSFC-610.2)[GEORGE MASON UNIVERSITY] > Cc: > wcs-1.2.swg@xxxxxxxxxxxxxxxxxxxxxxxx<mailto:wcs-1.2.swg@xxxxxxxxxxxxxxxxxxxxxxxx> > Subject: Re: [WCS-1.2.swg] Forecast times and dimensionality (domain axes) > > Hi all, > > Just thought I'd throw in an example where we use x/y/z/t1/t2 data. > > We serve a lot of forecast model data and John Caron has done a lot of > work to aggregate the data into 5D (2 time) cubes and then provide > various 4D slices through the 5D cube. The reason for the slices is to > allow tools that can't deal with a 5D(w/2t) cube to conveniently > interact with the data in various ways. > > Anyway, John put together a nice poster showing the "cube" and the > slices that I thought might be interesting input for this conversation. > Here's the link: > > http://www.unidata.ucar.edu/staff/caron/presentations/FmrcPoster.pdf > > Each slice is useful in a different way. For instance, the "Model Run" > slice is the traditional view for looking at forecasts. The "Constant > Valid Time" slice is useful for watching the evolution of the model > forecast as the forecast time gets closer. > > Also, in case anyone wants to look at some data, here is a link to all > the model data we serve > > http://motherlode.ucar.edu:8080/thredds/idd/models.html > > Each link leads to a 5D(w/2t) cube and the various slices. > > Ethan _______________________________________________ WCS-1.2.swg mailing list WCS-1.2.swg@xxxxxxxxxxxxxxxxxxxxxxxx<mailto:WCS-1.2.swg@xxxxxxxxxxxxxxxxxxxxxxxx> https://lists.opengeospatial.org/mailman/listinfo/wcs-1.2.swg
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