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

[Gökçen Güner]

Hi,
@Nikos:
I'll send information about maps tonight. I'll send 'r.describe -r' output
also.
@Sylvain:
Thank you, I didn't noticed the differences, my friend might had done
histogram strecthing over results, so this may be the reason for -2/2
thing, I will check this.
By the way I'm not sure how we can make data to have same layer extent and
dimension in both programs. I didn't do a special thing before calculation,
I just used g.region and
r.mapcalc methods.


2013/1/14 Sylvain Maillard <sylvain.maillard at gmail.com>

> 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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>>
>>
>
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