[GRASS-user] Differences Between ERDAS and GRASS About NDVI Statistics

[Gökçen Güner]

Hi Nikos,

Thank you for reply. I don't have exact NDVI band statistics but I have
statistics of NDVI difference maps. We calculated NDVI's of different times
for same area and then subtract one from another. I mean we have NDVI of
2000 and 2010 for the same area and we subtracted NDVI 2000 from NDVI 2010.
I can send statistics of original NDVI files tomorrow. These are statistics
of difference maps. I attached them to my post.

I'm not sure about 'different pixel inclusion rules' but I think
'spatial resolution
and extent' is same for both, you can see it in stat files I sent. Also we
didn't use MASK in both.
Thanks.

2013/1/14 Nikos Alexandris <nik at nikosalexandris.net>

> On Monday 14 of January 2013 01:03:42 Gökçen Güner wrote:
> > Hi,
>
> Hi!
>
> > This will not be a specific GRASS question. My friend and I tried to
> > calculate NDVIs of same data set and results seem same to us. But
> checking
> > statistics of each output we saw that statistics are very different. I
> > mean, median, mean, std.dev values of Grass output and Erdas output are
> > different.
>
> How different?  Can you please post exact numbers if that is ok? If
> differences
> are minor, you can simply ignore them :-)
>
> > We couldn't find an explanation for this situation. What can be
> > the reason for this?
>
> There are reasons. For example, different pixel inclusion rules (of course
> I
> have in mind border-near pixels). Other reasons might be the spatial
> resolution and extent (remember, GRASS-GIS is strict with that) and the
> presence of a MASK.
>
> The experts might give better clues.
>
> Best, Nikos
>
> > PS:
> > For NDVI, we used this formula: (NIR_Band - RED_Band) / (NIR_Band +
> > RED_Band)
>
> Note, grass7 has i.vi for this job :-)
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