invalid time issue in Aquarius SSS L4 products

invalid time issue in Aquarius SSS L4 products

Postby lmcgibbn » Tue Apr 03, 2018 9:20 am

Hi Folks,

I recently came across a metadata issue within the Aquarius SSS L4 products, an example of which can be found at https://opendap.jpl.nasa.gov/opendap/OceanWinds/aquarius/L4/IPRC/v4/7day/2015/148/SSS_OI_7D_20151482015154_V40.nc.html.

When I attempt to read the dataset using the super popular xarray (http://xarray.pydata.org/en/stable/generated/xarray.open_dataset.html) software, I am forced to use the
Code: Select all
decode_times=True
flag. This is due to the presence of the unit value within the time variable e.g.

Code: Select all
    float time(time=1);
      :long_name = "first day of the week over which Aquarius data have been collected";
      :units = "Julian days since December 31, 2010";
      :axis = "T";
      :_ChunkSizes = 1; // int


According to CF Conventions, the unit value is illegal

Code: Select all
Output of CF-Checker follows...

ut_are_convertible(): NULL unit argument

CHECKING NetCDF FILE: /tmp/5461.nc
=====================
WARNING: Cannot determine CF version from the Conventions attribute; checking against latest CF version: CF-1.6
Using CF Checker Version 2.0.9
Checking against CF Version CF-1.6
Using Standard Name Table Version 50 (2018-03-14T11:01:19Z)
Using Area Type Table Version 7 (14 March 2018)

WARNING (2.6.1): No 'Conventions' attribute present

------------------
Checking variable: latitude
------------------

------------------
Checking variable: sss
------------------
ERROR (3.1): Invalid units:  psu

------------------
Checking variable: longitude
------------------

------------------
Checking variable: time
------------------
ERROR (3.1): Invalid units:  Julian days since December 31, 2010
WARNING (4.4.1): Use of the calendar and/or month_lengths attributes is recommended for time coordinate variables
ERROR (4.4): Invalid units and/or reference time
 

ERRORS detected: 3
WARNINGS given: 3
INFORMATION messages: 0
lmcgibbn
 
Posts: 56
Joined: Wed Feb 17, 2016 9:36 pm

Re: invalid time issue in Aquarius SSS L4 products

Postby lmcgibbn » Tue Apr 03, 2018 11:26 am

From the Data Engineering team

This is definitely not CF compliant units for the Time variable.
The product is not 100% compliant unfortunately. The way around this and adjusting when integrating in services like THREDDS is via NCML, which allows one to flexibly augment metadata and address any key missing attributes or metadata errors.

For the future, fot sure if the provider plans on reprocessing for Aquarius v5.0 (this one is based on v4.0).
If so then, we will have them correct this and any other issues


As a heads up, if people are reading the product using xarray.open_dataset [0] then can do something like

Code: Select all
xarray.open_dataset(SSS_OI_7D_20151482015154_V40.nc, decode_times=True)


[0] http://xarray.pydata.org/en/stable/gene ... taset.html
lmcgibbn
 
Posts: 56
Joined: Wed Feb 17, 2016 9:36 pm


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