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Re: new changes to contour under IDL Version 3.1.1 (sunos sparc) [message #1498 is a reply to message #1302] Thu, 16 September 1993 09:43 Go to previous messageGo to previous message
thompson is currently offline  thompson
Messages: 584
Registered: August 1991
Senior Member
rfinch@water.ca.gov (Ralph Finch) writes:

> patrick> 1. Get rid of /MAX_VALUE. put in code to do real handling of
> patrick> missing data.

> Right on! I asked them about doing this some months ago but never got
> a reply.

I suspect that MAX_VALUE was put in to handle a specific project whose missing
data was all above a given value--perhaps Pioneer Venus, but I'm only guessing.
I know that it has been a part of CONTOUR for a long, long time.

Missing data can be handled in a number of ways, but I suspect that the system
I use is similar to what most people do. In my own IDL software, mainly
concerned with image display, I signal missing data values with a single
special value which can be set with the MISSING keyword. Actually the MISSING
keyword also shows up in some of the routines in the User's Library, such as
ROT or MAP_IMAGE.

FITS data files also allow for a single special value to signal missing data.
In the FITS header the particular special value for that data array is given by
the BLANK keyword. Pixels whose value is the same as that of BLANK are
considered to have no data.

If the special value signalling missing data is larger than any value in the
array, then it is consistent with IDL's MAX_VALUE keyword. However, it is
often more convenient to make the special flag value smaller than any possible
data value. For instance, in data arrays whose allowed values can only be
positive, I often set missing pixels to -1.

I think it would be a good idea if the IDL routines that currently accept the
MAX_VALUE keyword would also accept a MISSING keyword (or something like it) as
an alternative. That should be a relatively easy thing to do. Various
names for this keyword could be used, but I think MISSING would be the best,
because it would be consistent with routines already in the IDL User's Library.

Bill Thompson

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