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

[Sylvain Maillard]

Hi,

a quick look at the stats files show some differences:
- "Dimensions"
- "Layer Extent"
- not the same range of data (from -2 to 2 in ERDAS ? oO)

so that's totally logical to get different mean and stdev ...

take care to use exactly the same extent/res when you are comparing 2 files
;)


cheers,
Sylvain



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

> 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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