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Re: Iso-contours at maximum/minimum levels [message #64877 is a reply to message #64875]

Fri, 30 January 2009 07:04

Jeremy Bailin
Messages: 618
Registered: April 2008

Senior Member

On Jan 29, 6:14 pm, Paolo <pgri...@gmail.com> wrote:
> David Fanning wrote:
>> Gianluca Li Causi writes:
>
>>> In order to find the iso-value lines of a 2D surface I'm using the
>>> CONTOUR procedure, which works very well except that it's NOT ABLE to
>>> compute the contour when the LEVEL equals the maximum or minimum of
>>> the function.
>
>>> I've the following simple fuction:
>>> Z = shift(dist(100, 100), 50,50)
>>> Z = abs(Z - max(Z)*.3)
>>> which have a circular minimum at LEVEL=0, but CONTOUR is unable to
>>> find it!
>
> Well, just set your minimum level to min(z)+epsilon,
> where epsilon is a small number (i.e. 0.001 in this case),
> if you have a very flat bottom you need to contour out.
> (but please note that your z does not have a flat bottom,
> so it's a poor example...)
>
> Ciao,
> Paolo
>
>
>
>> According to my colleague Matt, what you should see at the
>> minimum of a contour plot is....nothing! I think I have to
>> agree. A contour line is suppose to enclose something.
>> What could be enclosed at the minimum value of a data set?
>> Right. Nothing.
>
>> You could hold a flat sheet of paper under your 2D surface
>> and draw a line where the surface touched the paper. But
>> the word for that would be an "etching" or an "imprint",
>> not a "contour". What you want, and what a contour plot
>> is designed to give, are two different things. Or at least
>> it seems that way to us. :-)
>
>> Cheers,
>
>> David
>
>> --
>> David Fanning, Ph.D.
>> Coyote's Guide to IDL Programming (www.dfanning.com)
>> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>
>

Paolo's suggestion makes perfect sense... but then I tried it and I'm
befuddled. The minimum contour consists of just a few dots!

There's something funny about how it's drawing the contour that gets
very confused by the abrupt change right at the minimum.

-Jeremy.

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[Message index]

		Re: Iso-contours at maximum/minimum levels By: pgrigis on Fri, 30 January 2009 11:00
		Re: Iso-contours at maximum/minimum levels By: David Fanning on Fri, 30 January 2009 07:48
		Re: Iso-contours at maximum/minimum levels By: pgrigis on Fri, 30 January 2009 07:36
		Re: Iso-contours at maximum/minimum levels By: David Fanning on Fri, 30 January 2009 07:21
		Re: Iso-contours at maximum/minimum levels By: Jeremy Bailin on Fri, 30 January 2009 07:04
		Re: Iso-contours at maximum/minimum levels By: pgrigis on Thu, 29 January 2009 15:14
		Re: Iso-contours at maximum/minimum levels By: David Fanning on Thu, 29 January 2009 14:51
		Re: Iso-contours at maximum/minimum levels By: David Fanning on Thu, 29 January 2009 08:15
		Re: Iso-contours at maximum/minimum levels By: Gianluca Li Causi on Mon, 02 February 2009 04:14

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