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

Tue Nov 17 04:56:56 PST 2015

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 <Joerg.Robl at 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 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> 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.
>

-- 
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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20151117/731e25cf/attachment.html>