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Re: Negative Variance? [message #65769 is a reply to message #65768]

Thu, 19 March 2009 18:47

anniebryant@gmail.com is currently offline

anniebryant@gmail.com
Messages: 16
Registered: March 2009

Junior Member

On Mar 19, 4:35 pm, "Jean H." <jghas...@DELTHIS.ucalgary.ANDTHIS.ca>
wrote:
> anniebry...@gmail.com wrote:
>> On Mar 19, 4:08 pm, Paolo <pgri...@gmail.com> wrote:
>>> anniebry...@gmail.com wrote:
>>>> I am using the VARIANCE comand in IDL and am getting negative values.
>>>> The description in the HELP menu for IDL doesn't provide much
>>>> information, but from what I know, I thought variance was either an
>>>> absolute value or a deviation from a squared number, neither of which
>>>> could yield a negative value.
>>>> I am looking at the variance for 2 different bands in an AVIRIS
>>>> image. I use the same .pro, just change the band I am looking at.
>>>> The results for Band 1 (lets call it) make sense: all positive
>>>> values. For Band 2, I get a lot of negatives.?! How is this
>>>> possible.
>>>> These are the values for Band 1:
>>>> integer mean std dev minimum maximum n_elements
>>>> 2633.3 2059.3 0.0000 9367.0 (160686) =
>>>> 160686
>>>> These are the values for Band2:
>>>> integer mean std dev minimum maximum n_elements
>>>> 1167.5 18269. -32768. 32767. (160686) =
>>>> 160686
>>> I don't see any variance here. The only negative number is the
>>> minimum.
>
>>> Ciao,
>>> Paolo
>
>> Those are the values of the vector AFTER the variance was computed, it
>> should read Variance_Band1 and Variance_Band2.
>
> How can you get multiple variances for a band?
> Jean

I created a moving kernel inside which to calculate the variance. So
the question is. What is the variance within a 17pixel x 17pixel
moving window. That is what I am calculating the variance for.

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[Message index]

		Re: Negative Variance? By: Homeyer on Thu, 19 March 2009 20:11
		Re: Negative Variance? By: wlandsman on Thu, 19 March 2009 19:43
		Re: Negative Variance? By: anniebryant@gmail.com on Thu, 19 March 2009 18:47
		Re: Negative Variance? By: Jean H. on Thu, 19 March 2009 15:35
		Re: Negative Variance? By: anniebryant@gmail.com on Thu, 19 March 2009 15:10
		Re: Negative Variance? By: pgrigis on Thu, 19 March 2009 15:08

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