[GRASS-user] r.univar: different results with different projections?

[Carlos Grohmann]

Hello Cesar

I wouldn't expect pixel resolution to play a role here. The statistics must
be carried out over the values.
The raster(s) I'm testing are rectangular, without null values. So
reprojecting will change the pixel size (from a square to a rectangle), but
it will (should) not change the number of pixels.

I'll try in another GRASS version and post the results.

best

Carlos



On Mon, Nov 16, 2015 at 4:48 PM, César Augusto Ramírez Franco <
caesarivs at gmail.com> wrote:

> Hello Carlos,
>
> Maybe it has something to do with the pixel resolution? I would certainly
> expect the same results, but with a very similar region (i.e. same number
> of rows and columns, or at least very close and a similar number of pixels)
>
> Maybe when projecting the raster, the length of the array is varying but I
> wouldn't expect to vary very much, are you getting the same results in a
> previous version of GRASS? (7.0 or even 6.4?)
>
> Greetings
> El nov. 16, 2015 11:35 AM, "Carlos Grohmann" <carlos.grohmann at gmail.com>
> escribió:
>
>> Hi all.
>>
>> I'm analyzing some global-scale DEMs, like ETOPO1/2, SRTM30_PLUS, etc.
>>
>> I'm getting the statistics for the whole dataset with r.univar, but today
>> I noticed that the results differ if I use different projections. (GRASS
>> 7.1)
>>
>> For example, ETOPO2:
>>
>> Using a latlong pseudo-projection (plate carré)
>>
>>
>> min: -10791
>> max: 8440
>> mean: -1888.6480792
>> median: -2457
>> stddev: 2650.01922839
>> p25: -4273
>> p75: 216
>>
>>
>> And using equal-area cylindrical:
>>
>> min: -10791
>> max: 8440
>> mean: -2379.78247747
>> median: -3291
>> stddev: 2507.89668617
>> p25: -4541
>> p75: 94
>>
>>
>>
>> Are these differences expected? I would think that they should be the
>> same, regardless of projection (I'm thinking on the elevation values as an
>> array of values).
>>
>> thanks
>>
>> Carlos
>>
>>
>>
>>
>>
>> --
>> Prof. Carlos Henrique Grohmann
>> Institute of Energy and Environment - Univ. of São Paulo, Brazil
>> - Digital Terrain Analysis | GIS | Remote Sensing -
>>
>> http://carlosgrohmann.com
>> http://orcid.org/0000-0001-5073-5572
>> ________________
>> Can’t stop the signal.
>>
>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>
>


-- 
Prof. Carlos Henrique Grohmann
Institute of Energy and Environment - Univ. of São Paulo, Brazil
- Digital Terrain Analysis | GIS | Remote Sensing -

http://carlosgrohmann.com
http://orcid.org/0000-0001-5073-5572
________________
Can’t stop the signal.
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