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Editorial Team
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Asked: 2026-06-18T15:37:29+00:00

While using the Unidata IDV to access the NOAA CO-OPS ocean model nowcast for

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While using the Unidata IDV to access the NOAA CO-OPS ocean model nowcast for Galveston Bay via OPeNDAP:

http://opendap.co-ops.nos.noaa.gov/thredds/catalog/NOAA/GBOFS/MODELS/201302/catalog.html?dataset=NOAA/GBOFS/MODELS/201302/nos.gbofs_hsc.fields.nowcast.20130201.t09z.nc

I get an error message “error creating data source”. It appears from their OPeNDAP Data Form that many of the NetCDF Climate and Forecast (CF) Metadata Conventions are met, but apparently not all.

Does anyone know what is missing or wrong?

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1 Answer

  1. Editorial Team

    You are correct, there are a few problems with this dataset that need to be fixed before it will work in IDV:

    1. The Conventions attribute says “COARDS”, but COARDS is appropriate for rectilinear grids (1D coordinate variables), and this data is curvilinear grid (2D lon,lat coordinate variables). The CF Conventions are appropriate here, so the “Conventions” global attribute need to be CF-1.0 or higher.

    2. To be CF-Compliant, the “coordinates” attribute needs to be specified for each variable.

    3. This model has a dimensionless vertical coordinate which requires the “positive” attribute to be “up”, and sigma values to be negative, not positive.

    Luckily, you can fix these problems using NcML. Try creating a file “gbofs.ncml” with the contents below and load that into IDV instead of the OPeNDAP URL. This approach will work with any software that uses the Unidata NetCDF-Java library, so it will work not only with IDV, but with Panoply, and NCTOOLBOX for Matlab.

    <?xml version="1.0" encoding="UTF-8"?>
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2"
  location="http://opendap.co-ops.nos.noaa.gov/thredds/dodsC/NOAA/GBOFS/MODELS/201302/nos.gbofs.fields.nowcast.20130201.t03z.nc">
  <!--Change conventions from COARDS to CF-1.6 -->
  <attribute name="Conventions" value="CF-1.6"/>
  <!--The "positive" attribute is only for the z coordinate variable, so remove it from depth and elevation-->
  <variable name="zeta">
    <remove type="attribute" name="positive"/>
  </variable>
  <variable name="depth">
    <remove type="attribute" name="positive"/>
  </variable>
  <!-- Dimensionless vertical coordinates need "positive"=up, and negative sigma values -->
  <variable name="sigma">
    <attribute name="positive" value="up"/>
    <values> -0.1667 -0.4167 -0.5833 -0.7643 -0.9167 </values>
  </variable>
  <!-- Curvilinear grids need a "coordinates" attribute that identifies the coordinate variables-->
  <variable name="temp" shape="time sigma ny nx" type="float">
    <attribute name="coordinates" value="lon lat sigma time"/>
  </variable>
</netcdf>

    I’ll report this to NOAA CO-OPS and hopefully they can fix this in the future. Here’s a screen shot from IDV just to show that this works:
    screen grab from IDV

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