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[Support #TOG-744630]: Grids from an OPeNDAP server

Hi Valentijn,

I put your email into our support system as I might need to pass this by 
someone else here and that will make it easier. Also, to get a response from 
whoever is available at the time, you might want to send questions to 
address@hidden or address@hidden rather than directly to me.

Valentijn Venus wrote:
> i Ethan,
> I'm running the "nightly" build of the IDV and if you select "I'm still
> feeling lucky" in Data Chooser / Catalog, when you are about to load the
> data, you will have the "Omni Control" as the only available option for
> displaying the data.
> However, if you select "Grids from an OPeNDAP server" instead, you will
> get the advanced from, where you are able to select (among others) the
> field needed, the spatial subset and the aggregation, which is exactly what 
> you want.
> I found that this works for the Pathfinder data. An example URL is:
> http://data.nodc.noaa.gov/cgi-bin/nph-dods/pathfinder/Version5.0/8day/2001/2001001-2001008.s0481pfv50-sst-16b.hdf
> It does not work for certain other OPeNDAP datasets, e.g. TOMS/EP:
> http://reason.gsfc.nasa.gov/opendap-bin/nph-dods/FTP_DATA/Giovanni/OPS/TOMS/EP/TOMS-EP_L3-TOMSEPL3_2005m0409_v8.HDF
> So here i have to "hardcode" the geographic/parameter subsetting at the end 
> of the url, for example:
> http://reason.gsfc.nasa.gov/opendap-bin/nph-dods/FTP_DATA/Giovanni/OPS/TOMS/EP/TOMS-EP_L3-TOMSEPL3_2005m0409_v8.HDF.dods?Erythemal[0:1:179][0:1:287]
> My main conclusion from this experiment has been that the so-called
> right-part of the URL (everything after the ?) should be generated by
> IDV. IDV has such provision, as long as you tell it that it is dealing
> with an OPeNDAP (DODS) server. The problem sometimes is that we have
> encountered an OPeNDAP server that is not compatible with IDV (like
> TOMS/EP). IDV seems to expect some parameters in the .das or .dds
> responses of DODS servers, which can not be found. 

It is definitely better to let the IDV handle construction of the query part of 
the URL if possible. I think the IDV can read a dataset with a query 
constraint. However, I don't think it can deal with any further subsetting on 
that dataset. OPeNDAP constraints don't combine easily and our code doesn't 
attempt to do so (or it might blindly do so, but then probably fails).

Yes, the IDV checks for certain conventions in the OPeNDAP metadata to 
determine how it should interpret the dataset.

> Q1: In such a case, can ncml "enhance" the way our THREDDS server
> interpretes the HDF/netCDF DODS server (e.g by reinterpreting the
> variables/units or by adding metadata) necessary for IDV properly
> recognize the gridded data and to show the spatial subsetting GUI? 

Yes, NcML can be used this way. I will try to put an example together for the 
Pathfinder URL you gave above and send that to you soon.

> Q2: Can you send me an example catalog that uses the datasetScan
> element for the Pathfinder SST OpenDAP server? I'd like to aggregate
> over multiple files but i'm not sure if the "JoinNew" / "Join Existing"
> element creates one virtual dataset e.g. all 8-day gridded SST's as a
> timeseries?

I'll have to take a look at that. Are the different datasets over the same 
spatial extent?

> Cheers, Valentijn


Ticket Details
Ticket ID: TOG-744630
Department: Support THREDDS
Priority: Normal
Status: Open

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