comp.lang.idl-pvwave archive: archive » Extraction phenological events

Home » Public Forums » archive » Extraction phenological events

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Extraction phenological events [message #80389]

Thu, 07 June 2012 10:52

Bandrush Barda
Messages: 6
Registered: June 2012

Junior Member

Dear all?
I’m doing phenological study using threshold method in IDL. I have 36
images with 10 days time interval in 2005
(ENVI format). They all have been stacked. I know how to set threshold
value on a single image. But I don’t know how to set threshold value
on multiband image to determine the date when grown season start and
end. Start of season is determined a day when NDVI value is return to
threshold value in a given year. So what I’d like to ask:
1. How can I set threshold value on multiband image to determine the
date when grown season start and end.
2. What kind of procedure and function should I use to extract start
and end of season using threshold method on multiband image? The dates
when start/end of season occurs are different in every vegetation
types. Basically I want to extract one image showing different places
are different phenological date. Is there anyone who can help me for
this?
Thanks a lot

Report message to a moderator

Re: Extraction phenological events [message #81113 is a reply to message #80389]

Thu, 16 August 2012 08:04

rogass
Messages: 200
Registered: April 2008

Senior Member

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
>
> images with 10 days time interval in 2005
>
> (ENVI format). They all have been stacked. I know how to set threshold
>
> value on a single image. But I don’t know how to set threshold value
>
> on multiband image to determine the date when grown season start and
>
> end. Start of season is determined a day when NDVI value is return to
>
> threshold value in a given year. So what I’d like to ask:
>
> 1. How can I set threshold value on multiband image to determine the
>
> date when grown season start and end.
>
> 2. What kind of procedure and function should I use to extract start
>
> and end of season using threshold method on multiband image? The dates
>
> when start/end of season occurs are different in every vegetation
>
> types. Basically I want to extract one image showing different places
>
> are different phenological date. Is there anyone who can help me for
>
> 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

Report message to a moderator

Re: Extraction phenological events [message #81884 is a reply to message #81113]

Sun, 28 October 2012 07:14

Dry in water
Messages: 18
Registered: October 2012

Junior Member

Dear all?

Thank you very much. It is very useful post and answers for my phenology study too. I want to make sure that different vegetation types have different thresholds. How to set these different thresholds corresponding to different vegetation types.

Thanks again

Report message to a moderator