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Frequently Asked Questions about NCEI NetCDF Templates

Should the content in each netCDF file be explicit to the file or the overall collection?

The intent of attributes is to describe the data contained within the netCDF file, not the entire dataset spanning multiple files.

Platforms which do not have call signs, IMO numbers, or WMO numbers; how should the Platform ID attribute be used?

If you are certain the WMO code does not exist, simply omit the wmo_code attribute or use the attribute wmo_code="Not applicable". If you are unsure if a code exists, use the following example format:

int platform_variable;
platform_variable:long_name = "Sea Spray" ;
platform_variable:comment = "Example platform container variable" ;
platform_variable:call_sign = "Unknown" ;
platform_variable:ices_code = "320D"; // ICES codes at: http://www.ices.dk/datacentre/requests/Login.aspx
platform_variable:wmo_code = "Unknown";//Information on getting WMO codes is available at http://www.wmo.int/pages/prog/amp/mmop/wmo-number-rules.html
platform_variable:imo_code = "Unknown";

Are Boolean flags required? or can you just have an enumerated flag?

The CF Conventions permit the use of boolean flag variables, enumerated flag variables, and a hybrid flag variable type that is a combination of the two. However, using the later case is incredibly difficult to explain and properly use. We recommend keeping them strictly enumerated or strictly boolean.

What if we have multiple instruments that recorded data on different sampling grids?

It is recommended that you separate the data from the instruments measuring at different frequencies into different netCDF files.

How do we determine what to input for the sea_name attribute?

We recommend that people use the actual sea name (example, "Labrador Sea" ) in the sea_name attribute. We are considering providing a web service for people to submit a coordinate and get all of the authoritative sea names that are affiliated with that area. However this will take some effort and we do not have a firm deadline in place yet.

What are 'accepted' fill values to use in the __FillValue attribute?

There is no single universally accepted fill value in use. However, if you are using the netCDF library to create your data, it will provide a default fill value for each data type (e.g. NC_FILL_CHAR, NC_FILL_BYTE, NC_FILL_SHORT, NC_FILL_INT, NC_FILL_FLOAT, and NC_FILL_DOUBLE in C). NCEI recommends the use of these defaults over the use of "nan" or "inf."

How do you define the geospatial_*_resolution attribute?

These attributes are an attempt to convey sample density not accuracy.

What is the significance of the global attribute uuid?

We are suggesting the inclusion of UUIDs to facilitate many possible future activities such as duplicate checking, version management, and rich inventories.

When defining the attribute 'coordinates' for geophysical_variable_n, does the order of the coordinate variables listed matter?

The order does not matter.

Software Debugging

Here is a list of software troubleshooting for different programs and compliance-checkers.

Ocean Data View (ODV)

Issue Explanation of Error Error Resolution
The data will not import into ODV ODV will not import data that is outside the specified minimum/maximum longitude, latitude coordinates Verify the lon/lat data is within the specified min/max.
ODV force quits when attempting to import data. ODV will force quit if data does not exist for the latitude longitude variables. Verify that there is actually longitude latitude data values in the variables.

ncBrowse

Issue Explanation of Error Error Resolution
ncBrowse is not plotting all of the data. ncBrowse will not plot data which is outside the valid_min/max values (invalid data). Verify the valid_min/max values for that variable are outside the minimum/maximum values.
The data plotting is not the values listed in the file. ncBrowse automatically evaluates the values based on the specifications of scale_factor and add_offset. Check the geophysical_variable:scale_factor and geophysical_variable:add_offset values.

Matlab

Issue Explanation of Error Error Resolution
     

CF-Compliance Checkers

Error Message Explanation of Error Error Resolution
ERROR: Attribute valid_min/max of incorrect type valid_min/max is the wrong type of variable adjust the variable to ncdouble or ncfloat
ERROR (3.5): Number of flag_values values must equal the number or words/phrases in flag_meanings There must be x number of flag_values corresponding to x number of flag_meanings adjust the amount of flag_values or flag_meanings
ERROR: Attribute flag_values of incorrect type The list of flag_values needs to be of the correct type Adjust the values in flag_values to ncint.