| Re: Extraction phenological events [message #81113 is a reply to message #80389] |
Thu, 16 August 2012 08:04   |
rogass
Messages: 200
Registered: April 2008
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Senior Member |
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Am Donnerstag, 7. Juni 2012 19:52:38 UTC+2 schrieb Bandrush Barda:
> Dear all?
>
> I’m doing phenological study using threshold method in IDL. I have 36
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> images with 10 days time interval in 2005
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> (ENVI format). They all have been stacked. I know how to set threshold
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> value on a single image. But I don’t know how to set threshold value
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> on multiband image to determine the date when grown season start and
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> end. Start of season is determined a day when NDVI value is return to
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> threshold value in a given year. So what I’d like to ask:
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> 1. How can I set threshold value on multiband image to determine the
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> date when grown season start and end.
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> 2. What kind of procedure and function should I use to extract start
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> and end of season using threshold method on multiband image? The dates
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> when start/end of season occurs are different in every vegetation
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> types. Basically I want to extract one image showing different places
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> are different phenological date. Is there anyone who can help me for
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> this?
>
> Thanks a lot
Hi,
as far as I understood you have a stack of NDVI images. If then I would transfer them to IDL by the export function in the ENVI file menu. then I would apply this:
;stack is the name of the varaible you have used for export
sz=size(stack,/dim)
threshold=0.3; you must set this
stack ge= threshold
wh=where(~stack,c)
if c gt 0 then stack[wh]=!values.f_nan
stack*=rebin(lindgen(1,1,sz[2]),sz)
startperiod=min(stack,max=endperiod,/nan,dimension=3)
startperiod and endperiod are then two matrices of your spatial extent containing the frame number per pixel when the period hast started and ended due to the selected threshold. All elements of the matrices having NAN didn't pass the selected threshold.
Cheers
CR
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