Depending on how much you trust the resampling methods, both are "right" in their projection system and using this resampling method. 
Look at your resolution: in the projected systems your pixels are strongly rectangular:
This makes a difference as to which pixels are seen to be the nearest neighbors. By displaying the map in both systems side by side and by zooming very closely you can see this quite clearly. Pixels are not equal to pixels when moving from one projection system to another. The grid you lay over the earth is different and so the statistics calculated from that grid will be different as well.
Have a look at the attached screenshot. You can see the etopo data on the coast of greenland in EPSG 4326 and in a "World Mercator" system (+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +no_defs +a=6378137 +rf=298.257223563 +towgs84=0.000,0.000,0.000 +to_meter=1).
The values I got was from the right-hand land pixel just above the central water area. As you see values are different, as these pixels are not equal.
Reprojection of raster maps is not the same as reprojection of vector maps in which you reproject the vertices of an object, but the attributes of that object remain the same. As explained in the r.proj manual:
"A raster may be considered o represent a sampling of a process at a regular, ordered set of locations. The set of locations that lie at the intersections of a cartesian grid in one projection will not, in general, coincide with the sample points in another projection. Thus, the conversion of raster maps involves an interpolation step in which the values of points at intermediate locations relative to the source grid are estimated."
On Tue, Nov 17, 2015 at 12:50 PM, Anna Petrášová <kratochanna@gmail.com
<mailto:kratochanna@gmail.com>> wrote:
On Tue, Nov 17, 2015 at 7:56 AM, Carlos Grohmann
<carlos.grohmann@gmail.com <mailto:carlos.grohmann@gmail.com>> wrote:
Hello Jörg
The area of cell shouldn't influence here. The statistics are
about the elevation values, regardless of the area represented
by pixel. If I think on the pixels as equally-spaced vector
points, after projection they won't be equally-spaced anymore,
but the number of points won't change. So the mean of their
values (and stddev, etc) shouldn't change as well.
I don't think you can treat pixels as vector points here, I agree
with what Jörg was saying. If some vector points would get further
away, you will get new pixels in between and if the points get close
enough, the information in all points but one is lost. This is at
least my intuitive understanding of the NN reprojection which can be
wrong. I wouldn't be concerned that the results changed but how much
they changed.
Anna
regards
Carlos
On Tue, Nov 17, 2015 at 5:15 AM, Robl Jörg Christian
<Joerg.Robl@sbg.ac.at <mailto:Joerg.Robl@sbg.ac.at>> wrote:
Dear Carlos,____
__ __
I’m not an expert for projections. ____
However, on Lat/Long WGS84 the actual area of cells decline
from the equator towards the poles. ____
Thus, I would expect that cell values near the poles have
“more weight” using Lat/Long WGS84 than using an equal area
projection. ____
__ __
Near the poles I don’t understand how the values for extent
and resolution should be correct (equal area), except there
is a huge distortion (very likely for a cylindrical
projection)!____
Are there really 21600 cols with a nsres = 1178 m at the
north and south pole. I would call this a huge distortion.____
__ __
As a test, I would calculate the statistics for a smaller
area centered at the equator. I would expect that the
results are very similar comparing the lat/long and the
reprojected dataset.____
__ __
Regards Jörg____
__ __
__ __
__ __
*Von:*grass-user [mailto:grass-user-bounces@lists.osgeo.org
<mailto:grass-user-bounces@lists.osgeo.org>] *Im Auftrag von
*Carlos Grohmann
*Gesendet:* Montag, 16. November 2015 23:32
*Cc:* GRASS user list
*Betreff:* Re: [GRASS-user] r.univar: different results with
different projections?____
__ __
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@gmail.com <mailto:caesarivs@gmail.com>>
wrote:____
Carlos,____
__ __
2015-11-16 14:47 GMT-05:00 Carlos Grohmann
<carlos.grohmann@gmail.com
<mailto:carlos.grohmann@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.____
--
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@lists.osgeo.org <mailto:grass-user@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.
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
