Problem with MATLAB and Earthdata OPeNDAP

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benedettobarone
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Problem with MATLAB and Earthdata OPeNDAP

by benedettobarone » Thu May 11, 2023 8:16 pm America/New_York

I am trying to access the Earthdata OPeNDAP repository for a sea surface temperature product (OSPO Global Nighttime Foundation Sea Surface Temperature) that I previously downloaded from the PODACC OPeNDAP. While I did not have any issue with PODACC, I cannot access the Earthdata repository. For example, I tried to use the ncinfo comand to access the following granule:

https://opendap.earthdata.nasa.gov/collections/C2036877745-POCLOUD/granules/20230510000000-OSPO-L4_GHRSST-SSTfnd-Geo_Polar_Blended_Night-GLOB-v02.0-fv01.0

And I obtained the following error:
"The NetCDF library encountered an error during execution of 'open' function - 'Access failure (-77)'."

I have the Earthdata login in my .netrc and last year I used it to download ocean currents from the OSCAR data product. Today, I could access those data after one attempt, but when trying again I obtained the following error:
"The NetCDF library encountered an error during execution of 'open' function - 'DAP server error (NC_EDAPSVC)'"

Lastly, I tried the same approach with the two servers reported in the MATLAB tutorial (https://opendap.github.io/documentation/tutorials/matlab_tutorial.html) and I could not access those data either. This time, I obtained the following error:
"The NetCDF library encountered an error during execution of 'open' function - 'Not a valid data type or _FillValue type mismatch (NC_EBADTYPE)'"

I do not have problems using MATLAB to access other OPeNDAP servers (the Copernicus ones, for example). I am wondering if anyone knows the cause of this problem and/or if anyone is having similar issues using MATLAB and the EarthData OPeNDAP. Any help is appreciated.

PS: I am working on a MAC with OS Ventura and MATLAB R2021a

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OB ODPS - towens
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Re: Problem with MATLAB and Earthdata OPeNDAP

by OB ODPS - towens » Tue May 16, 2023 9:52 am America/New_York

Try accessing the OPeNDAP URL with a fresh web browser window. See if there is a user authentication step that Matlab is not handling.

Tommy

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Re: Problem with MATLAB and Earthdata OPeNDAP

by benedettobarone » Tue May 16, 2023 5:04 pm America/New_York

Thanks for your help, Tommy,
If I restart the browser and connect to the OPeNDAP URL, it asks for my Earthdata login. Is the browser also supposed to obtain this information from the .netrc?
After the login, I have normal access to the URL.

Benedetto

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Re: Problem with MATLAB and Earthdata OPeNDAP

by OB ODPS - towens » Wed May 17, 2023 10:53 am America/New_York

No - the browser usually has its own cache of saved passwords. But this was just a way to demonstrate the need for authentication. I am not a Matlab user, but hopefully someone else can walk you through the steps for obtaining and using an access token in Matlab.

Tommy

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Re: Problem with MATLAB and Earthdata OPeNDAP

by PODAAC - colleen » Wed May 17, 2023 4:32 pm America/New_York

We are sorry to hear that you are experiencing problems while trying to access the GHRSST data.

Our SME tried the OPeNDAP URL referenced today: https://opendap.earthdata.nasa.gov/collections/C2036877745-POCLOUD/granules/20230510000000-OSPO-L4_GHRSST-SSTfnd-Geo_Polar_Blended_Night-GLOB-v02.0-fv01.0

She accessed it using her browser and wget, and was able to successfully do a direct download in nc4. However, she had to refresh her requests twice because it was encountering a server issue. The Hyrax team is aware of BES connection issue (it’s an issue with their production env), but we are unsure what their timeline is to fix it.

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Re: Problem with MATLAB and Earthdata OPeNDAP

by dwhitt25 » Fri Jun 23, 2023 6:33 pm America/New_York

Today, I unsuccessfully tried several different ways to access podaac data via opendap using Matlab 2021a on MacOS Monterey, including https://opendap.earthdata.nasa.gov/collections/C2270392799-POCLOUD/granules/ssh_grids_v2205_1993020712.dap.nc4, and the data originally referenced in the initial post of this thread.

In both cases, I am able to download the data from the opendap url using safari, chrome, and curl in the command line, similar to the last post above.

But, I cannot get access in Matlab. Like the original poster, I can access opendap files using Matlab, e.g. ncdisp('http://test.opendap.org/dap/data/nc/sst.mnmean.nc.gz'), but not those in earthdata.nasa.gov.

I also cannot get the dap4:// protocol to work in the tutorial linked in the original post.

I spent a lot of time trying to get a .dodsrc file right, but I'm not certain that it is right, and I don't think I have another netcdf/C library program to test on this machine.

Do any Matlab users know a tutorial that works through the authentication/access issues for Matlab/Opendap?

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Re: Problem with MATLAB and Earthdata OPeNDAP

by PODAAC - wenhaoli » Tue Jun 27, 2023 5:37 pm America/New_York

I have tried the wget command on linux machine. It works for both nc files, see commands below.
% wget --http-user=username --ask-password --keep-session-cookies https://opendap.earthdata.nasa.gov/collections/C2270392799-POCLOUD/granules/ssh_grids_v2205_1993020712.dap.nc4
% wget --http-user=username --ask-password --keep-session-cookies https://opendap.earthdata.nasa.gov/collections/C2036877745-POCLOUD/granules/20230510000000-OSPO-L4_GHRSST-SSTfnd-Geo_Polar_Blended_Night-GLOB-v02.0-fv01.0

Regarding the Matlab program, you could try to use the webread() function. It will return a long list of uint8. You probably need to convert it to a structure.

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