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

[Nikos Alexandris]

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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