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

[Nikos Alexandris]

Gökçen Güner:

> > 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.

What kind of images are those, if I may ask? Which sensor?

> > 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.

> > 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.

Also,

what about NULL or NoData cells? How does ERDAS deal with them -- I don't 
remember now. How are they defined, if they exist, in your image?

What does "r.describe -r" return for the data set(s) in question?

Thanks, Nikos
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