Dear Jorge,
thank you very much for the help. i have used the same landsat package 
but did not convert on the ground reflection but the on sensor reflectance.
For MODIS and AVHRR, i need better spatial resolution that is the reason 
i am using landsat and I was not getting negative values even places 
where there is no vegetation instead getting higher values like 0.2-0.4 
which is quite high for non vegetated area. Now i am trying to get NDVI 
from surface reflectance.

Regards,

Vikram.

On 1/18/2012 2:26 PM, Jorge Cabrera wrote:
> Dear Vikram,
> I do not know which procedure you used for transforming the values 
> from DN to reflectivity. But anyway I would like to recommend to use 
> the landsat package or take a look on it.
> www.jstatsoft.org/v43/i04/paper
> The values of NDVI should be negative where there is no vegetation. 
> Also for getting a better idea of the phenological cycle you can use 
> MODIS data to sample ndvi changes within a longer period with more 
> temporal resolution.
>
> I hope this help.
>
> Best,
>
> Jorge
>
>
> ------------------------------------------------------------------------
> *From:* Vikram Ranga <babuawara@gmail.com>
> *To:* r-sig-geo@r-project.org
> *Sent:* Wednesday, January 18, 2012 10:49 AM
> *Subject:* [R-sig-Geo] problem with ndvi
>
> Hello list,
> I am working with landsat images and would like to create a 
> phenological cycle with NDVI.
> I am using R for that but NDVI values changes if i use on sensor 
> reflectance instead of DN values i.e. giving different result in every 
> case.
> I tried to compare histograms seems to have changed and I am not 
> getting negative values where there is no vegetation but if i use DN 
> values it gives negative values to such locations. Do anybody have any 
> idea what went wrong??
>
> Thanks in advance and would be much appreciated.
>
> Vikram
>
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