Extraction phenological events

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

Dear Bandrush Barda,

To my knowledge, there is no way to do this from the standard ENVI interface. The point is that you are not interested in the values themselves, but rather the point in time where they are occurring. This can be extracted easily by programming a script in IDL, but not from the ENVI interface. Have alook at my implementation of HANTS (Harmonic Analysis of NDVI Time Series)as an example of how to process time-series in IDL/ENVI. You can find HANTS in the code library, just search with the keyword HANTS and it will be your first hit.

with best regards,

Allard de Wit

HI Allard,

I was just wondering if you could maybe offer some tips on how to use the HANTS algorithm in ENVI IDL 4.7? I'm new to IDL and it all seems to be quiteoverwhelming. I'm attempting to use HANTS to denoise the NDVIs of 16-Day L3 Global 250 m datasets (short name MOD13Q1) but don't understand how to implement the *.pro files included in the HANTS zip file. Is this even suitable/possible?

I would be very grateful for any help you can offer,

David Lee

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'tpass the selected threshold.

Cheers

CR