[R-sig-Geo] Antw: problem with ndvi

Agustin Lobo alobolistas at gmail.com
Thu Jan 19 11:26:54 CET 2012


Matteo,
by "RemoteSensing package" you mean the landsat package?
Agus

2012/1/18 Matteo Mattiuzzi <matteo.mattiuzzi at boku.ac.at>:
>
> Dear Vikram,
>
> It is not good to calculate NDVI from DN. The purpose of DN (digital
> number) is only for packing as much information as possible into a given
> “BIT-size” (in Landsat 8-bit/pixel/band).
> DN is practically the radiance (I think TOA?) modified with an offset
> and a gain in order to press or expand the range between 0 and 255.
> This means that the relation between bands is changed, and this is a
> crucial need for NDVI (and generally for indices).
> Probably the minimum pre-processing you absolutely need to do is the
> conversion from DN to RADIANCE (TOA) using the gain and bias information
> in the MTL file (metadata). Better and more precise is to calculate the
> ground reflectance. I did this with the RemoteSensing package on
> R-Forge. But the current version doesn't work because of incomplete
> modifications in the package code.
>
> Non-vegetated areas can have positive values. I think the soil has
> often a NDVI of 0.1. Clouds sometimes have even 0.2 as...anyway you
> can't trust a NDVI from DN and you can't sharply differentiate
> veg/nonVeg by positive or negative ndvi values. Water normally has a
> clear negative NDVI (~-0.2).
>
> Also implemented in the RemoteSensing package are functions to compute
> other vegetation indices, I did not test them but maybe you can find
> something useful there (type “?ndvi” after installing and loading the
> package).
>
> Cheers Matteo
>
>
>
>
>>>> Vikram Ranga <babuawara at gmail.com> 1/18/2012 10:49 am >>>
> 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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