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

[Carlos Grohmann]

Hello Cesar

That was weird, so I tested it again. The number of cells is the same for
both projections, but the values differ. This must be related to
reprojecting.

To me, they shouldn't de different, since a nearest neighbor should
preserve the original values. I'm not really comfortable with this, as I'm
not sure I can trust the stats after projecting.

best

Carlos


GRASS 7.1.svn (latlong):~ > g.region raster=gdem_etopo1_ice -pa
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      90N
south:      90S
west:       180W
east:       180E
nsres:      0:01
ewres:      0:01
rows:       10800
cols:       21600
cells:      233280000
GRASS 7.1.svn (latlong):~ > r.univar map=gdem_etopo1_ice -ge percentile=100
n=233280000
null_cells=0
cells=233280000
min=-10803
max=8333
range=19136
mean=-1892.40422534294
mean_of_abs=2644.91906490912
stddev=2649.98339302808
variance=7022411.98332463
coeff_var=-140.032629262802
sum=-441460057688
first_quartile=-4286
median=-2457
third_quartile=214
percentile_100=8333




GRASS 7.1.svn (eqarea):~ > g.region -p
projection: 99 (Equal Area Cylindrical)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      6363885.33192604
south:      -6363885.33192604
west:       -20037508.34278924
east:       20037508.34278924
nsres:      1178.49728369
ewres:      1855.32484655
rows:       10800
cols:       21600
cells:      233280000
GRASS 7.1.svn (eqarea):~ > r.univar map=gdem_etopo1_ice -ge percentile=100
n=233280000
null_cells=0
cells=233280000
min=-10803
max=8333
range=19136
mean=-2382.28934158093
mean_of_abs=2845.10169015775
stddev=2508.93105538271
variance=6294735.0406638
coeff_var=-105.315966939504
sum=-555740457604
first_quartile=-4544
median=-3285
third_quartile=93
percentile_100=8333







On Mon, Nov 16, 2015 at 7:43 PM, César Augusto Ramírez Franco <
caesarivs at gmail.com> wrote:

> Carlos,
>
> 2015-11-16 14:47 GMT-05:00 Carlos Grohmann <carlos.grohmann at gmail.com>:
>
>> GRASS 7.1.svn (base_maps):~ > g.region -p raster=gdem_etopo1_ice
>> cells:      233280000
>>
>> GRASS 7.1.svn (base_maps):~ > r.univar map=gdem_etopo1_ice -ge
>> percentile=100
>> cells=58320000
>>
>> GRASS 7.1.svn (eqarea):~ > r.univar map=gdem_etopo1_ice -ge percentile=100
>> cells=233280000
>>
>
> ​​Notice how the number of pixels differs, that's the reason the
> statistics are not the same​, I don't get why the region has a different
> number of pixels than the raster itself in the original latlong
> projection...​ I think that's the root of the issue
>
> --
> *César Augusto Ramírez Franco*
> Laboratorio de Sistemas Complejos Naturales
> Escuela de Geociencias - Facultad de Ciencias
> Universidad Nacional de Colombia - Sede Medellín
> Teléfono: (57-4) 430 9369 - 300 459 6085
> http://labscn-unalmed.github.io/
>



-- 
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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