Missing data

I am having a little bit of trouble representing my data set how I would like
to; ideally, certain instances of a flight path (lat, lon, alt) should be set
as "missing".  I found the following in the VisAD documentation: 

        Any Data object may take the value 'missing', and any sub-object of a 
        object may take the value 'missing'. 

Unfortunately, I can't seem to figure out exactly how to set the value as 
missing.  I tried to find a method that would do this for me, and the 
closest thing I could come up with was missingData(). a MathType method.  
I'm assuming that since I want to declare a RealType as missing that this 
method is of no use to me?

Incidentally, I am not even sure as to which stage in the development of 
the Data I should go back to to set the data as missing.  Is it possible 
to manipulate the DataReference?  Perhaps it's a method to manipulate the 
FlatField that I'm looking for?  Maybe (as per usual), I'm just 
completely off-base with any of my hunches?

Many thanks,
Vikram Vatsa <v.vatsa@xxxxxxxxxxxxx>
Computer Sciences Corporation