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.
regards Carlos On Tue, Nov 17, 2015 at 5:15 AM, Robl Jörg Christian <[email protected]> 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:[email protected]] *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 < > [email protected]> wrote: > > Carlos, > > > > 2015-11-16 14:47 GMT-05:00 Carlos Grohmann <[email protected]>: > > 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.
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