Null values in Linear2DSet?

I am working with elevation data that can be very nicely held
in a Linear2DSet, except that sometimes a few of the values
are missing around the edges so that the rectangular area is
not completely filled.

What is the best way of handling this? Can I set a null
value? Or do I have to go to am Irregular2DSet? What would be
the performance penalty of doing so?

Sorry, I did have a look through the email archives and test
examples but could not find an answer.

Cheers,

Andrew Wilson
BG Group

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