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

[Robl Jörg Christian]

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 at 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 at gmail.com<mailto:caesarivs at gmail.com>> wrote:
Carlos,

2015-11-16 14:47 GMT-05:00 Carlos Grohmann <carlos.grohmann at gmail.com<mailto: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://carlosgrohmann.com/>
http://orcid.org/0000-0001-5073-5572
________________
Can’t stop the signal.
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