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Some sort of NULL value in IDL ?
| Re: Some sort of NULL value in IDL ? [message #20708 is a reply to message #20674] |
Tue, 18 July 2000 00:00  |
Martin Schultz
Messages: 515
Registered: August 1997
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Senior Member |
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"Liam E.Gumley" wrote:
>
> deckard_007@my-deja.com wrote:
>> Is there something like a NULL value available in IDL ? I have some data
>> which I have to export as a HDF file. But not all of my data is good,
>> so I want to export the bad points as NULL values like in a database.
>> Maybe this has more to do with the HDF format than with IDL itself, but
>> maybe someone has encountered this problem before.
>
> This issue is handled in HDF Scientific Datasets by attaching standard
> attributes to an SDS variable. The following attributes have been
> established by the HDF and netCDF communities:
>
> long_name
> A text string which describes a variable in detail (e.g. 'Northwards
> velocity component').
>
> units
> A text string which describes the units of a variable (e.g.
> 'meters/second').
>
> valid_range
> A two element array containing the valid minimum and maximum values for
> a variable (e.g. ). The type of this attribute should be the
> same as the variable type.
>
> scale_factor
> A multiplier to be applied to a variable data after it is read (allows
> float values to be stored as short or byte). The type of this attribute
> should be the same as the desired unscaled variable type (e.g. float).
>
> add_offset
> An offset to be added to a variable after it is read, and after
> scale_factor (if present) is applied. The type of this attribute should
> be the same as the desired unscaled variable type (e.g. float).
>
> _FillValue
> A value indicating that data was not written. The type of this attribute
> should be the same as the variable type.
>
> In your case, the _FillValue attribute could be used to indicate missing
> data.
>
> Cheers,
> Liam.
> http://cimss.ssec.wisc.edu/~gumley
... isn't there yet another standard attribute named missing_value which
would be even more appropriate?
Martin
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