Return the default value for an unset standard CF coordinate conversion term.
The default values are stored in the file cf/etc/coordinate_reference/default_values.txt.
| Parameters: |
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|---|---|
| Returns: |
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| Examples: |
>>> cf.CoordinateReference.default_value('ptop')
0.0
>>> print cf.CoordinateReference.default_value('north_pole_grid_latitude')
None