[gradsusr] Setting missing values (or any value) to undefined

[Matt Bunkers - WFO UNR]

Hi,

I have hourly wind chill (WC) grids for which I want to find the minimum
WC.  The problem I have is that some of the areas of the grids are missing
(set to -120).  When I compute the minimum WC and then display the result
(as either image or contour) I get huge gradients between the real data and
the missing data.  What I would like to do is set my missing data to
undefined (so they don't plot).  I've tried that with the const function
but that doesn't appear to work.  Is there another function that would
allow me to set a given value (-120 in this case) of my grid to undefined?
If it is set to undefined then when I plot the grid I shouldn't see those
horrendous gradients.

Thanks,

-- Matt
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