<div dir="ltr">Hello Jörg<div><br></div><div>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.</div><div><br></div><div>regards</div><div><br></div><div>Carlos</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Nov 17, 2015 at 5:15 AM, Robl Jörg Christian <span dir="ltr"><<a href="mailto:Joerg.Robl@sbg.ac.at" target="_blank">Joerg.Robl@sbg.ac.at</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Dear Carlos,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I’m not an expert for projections.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">However, on Lat/Long WGS84 the actual area of cells decline from the equator towards the poles.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Thus, I would expect that cell values near the poles have “more weight” using Lat/Long WGS84 than using an equal area projection.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">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)!<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Are there really 21600 cols with a nsres = 1178 m at the north and south pole. I would call this a huge distortion.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">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.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Regards Jörg<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><b><span lang="DE" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Von:</span></b><span lang="DE" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> grass-user [mailto:<a href="mailto:grass-user-bounces@lists.osgeo.org" target="_blank">grass-user-bounces@lists.osgeo.org</a>]
<b>Im Auftrag von </b>Carlos Grohmann<br>
<b>Gesendet:</b> Montag, 16. November 2015 23:32<br>
<b>Cc:</b> GRASS user list<br>
<b>Betreff:</b> Re: [GRASS-user] r.univar: different results with different projections?<u></u><u></u></span></p><div><div class="h5">
<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Hello Cesar<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">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. <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">best<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Carlos<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">GRASS 7.1.svn (latlong):~ > g.region raster=gdem_etopo1_ice -pa<u></u><u></u></p>
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<p class="MsoNormal">projection: 3 (Latitude-Longitude)<u></u><u></u></p>
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<p class="MsoNormal">zone: 0<u></u><u></u></p>
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<p class="MsoNormal">datum: wgs84<u></u><u></u></p>
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<p class="MsoNormal">ellipsoid: wgs84<u></u><u></u></p>
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<p class="MsoNormal">north: 90N<u></u><u></u></p>
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<p class="MsoNormal">south: 90S<u></u><u></u></p>
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<p class="MsoNormal">west: 180W<u></u><u></u></p>
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<p class="MsoNormal">east: 180E<u></u><u></u></p>
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<p class="MsoNormal">nsres: 0:01<u></u><u></u></p>
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<p class="MsoNormal">ewres: 0:01<u></u><u></u></p>
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<p class="MsoNormal">rows: 10800<u></u><u></u></p>
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<p class="MsoNormal">cols: 21600<u></u><u></u></p>
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<p class="MsoNormal">cells: 233280000<u></u><u></u></p>
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<p class="MsoNormal">GRASS 7.1.svn (latlong):~ > r.univar map=gdem_etopo1_ice -ge percentile=100<u></u><u></u></p>
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<p class="MsoNormal">n=233280000<u></u><u></u></p>
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<p class="MsoNormal">null_cells=0<u></u><u></u></p>
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<p class="MsoNormal">cells=233280000<u></u><u></u></p>
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<p class="MsoNormal">min=-10803<u></u><u></u></p>
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<p class="MsoNormal">max=8333<u></u><u></u></p>
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<p class="MsoNormal">range=19136<u></u><u></u></p>
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<p class="MsoNormal">mean=-1892.40422534294<u></u><u></u></p>
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<p class="MsoNormal">mean_of_abs=2644.91906490912<u></u><u></u></p>
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<p class="MsoNormal">stddev=2649.98339302808<u></u><u></u></p>
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<p class="MsoNormal">variance=7022411.98332463<u></u><u></u></p>
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<p class="MsoNormal">coeff_var=-140.032629262802<u></u><u></u></p>
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<p class="MsoNormal">sum=-441460057688<u></u><u></u></p>
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<p class="MsoNormal">first_quartile=-4286<u></u><u></u></p>
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<p class="MsoNormal">median=-2457<u></u><u></u></p>
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<p class="MsoNormal">third_quartile=214<u></u><u></u></p>
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<p class="MsoNormal">percentile_100=8333<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">GRASS 7.1.svn (eqarea):~ > g.region -p<u></u><u></u></p>
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<p class="MsoNormal">projection: 99 (Equal Area Cylindrical)<u></u><u></u></p>
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<p class="MsoNormal">zone: 0<u></u><u></u></p>
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<p class="MsoNormal">datum: wgs84<u></u><u></u></p>
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<p class="MsoNormal">ellipsoid: wgs84<u></u><u></u></p>
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<div>
<p class="MsoNormal">north: 6363885.33192604<u></u><u></u></p>
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<div>
<p class="MsoNormal">south: -6363885.33192604<u></u><u></u></p>
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<p class="MsoNormal">west: -20037508.34278924<u></u><u></u></p>
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<p class="MsoNormal">east: 20037508.34278924<u></u><u></u></p>
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<p class="MsoNormal">nsres: 1178.49728369<u></u><u></u></p>
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<p class="MsoNormal">ewres: 1855.32484655<u></u><u></u></p>
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<p class="MsoNormal">rows: 10800<u></u><u></u></p>
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<p class="MsoNormal">cols: 21600<u></u><u></u></p>
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<p class="MsoNormal">cells: 233280000<u></u><u></u></p>
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<p class="MsoNormal">GRASS 7.1.svn (eqarea):~ > r.univar map=gdem_etopo1_ice -ge percentile=100 <u></u><u></u></p>
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<p class="MsoNormal">n=233280000<u></u><u></u></p>
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<p class="MsoNormal">null_cells=0<u></u><u></u></p>
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<p class="MsoNormal">cells=233280000<u></u><u></u></p>
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<p class="MsoNormal">min=-10803<u></u><u></u></p>
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<p class="MsoNormal">max=8333<u></u><u></u></p>
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<p class="MsoNormal">range=19136<u></u><u></u></p>
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<p class="MsoNormal">mean=-2382.28934158093<u></u><u></u></p>
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<p class="MsoNormal">mean_of_abs=2845.10169015775<u></u><u></u></p>
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<p class="MsoNormal">stddev=2508.93105538271<u></u><u></u></p>
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<p class="MsoNormal">variance=6294735.0406638<u></u><u></u></p>
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<p class="MsoNormal">coeff_var=-105.315966939504<u></u><u></u></p>
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<p class="MsoNormal">sum=-555740457604<u></u><u></u></p>
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<div>
<p class="MsoNormal">first_quartile=-4544<u></u><u></u></p>
</div>
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<p class="MsoNormal">median=-3285<u></u><u></u></p>
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<p class="MsoNormal">third_quartile=93<u></u><u></u></p>
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<p class="MsoNormal">percentile_100=8333<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">On Mon, Nov 16, 2015 at 7:43 PM, César Augusto Ramírez Franco <<a href="mailto:caesarivs@gmail.com" target="_blank">caesarivs@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal"><span style="color:#274e13">Carlos,<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">2015-11-16 14:47 GMT-05:00 Carlos Grohmann <<a href="mailto:carlos.grohmann@gmail.com" target="_blank">carlos.grohmann@gmail.com</a>>:<u></u><u></u></p>
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<div>
<div>
<p class="MsoNormal">GRASS 7.1.svn (base_maps):~ > g.region -p raster=gdem_etopo1_ice<u></u><u></u></p>
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<div>
<p class="MsoNormal">cells: 233280000<u></u><u></u></p>
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<div>
<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">GRASS 7.1.svn (base_maps):~ > r.univar map=gdem_etopo1_ice -ge percentile=100<u></u><u></u></p>
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<p class="MsoNormal">cells=58320000<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<div>
<p class="MsoNormal">GRASS 7.1.svn (eqarea):~ > r.univar map=gdem_etopo1_ice -ge percentile=100<u></u><u></u></p>
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<p class="MsoNormal">cells=233280000<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><span style="color:#274e13">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<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="color:#888888"><u></u> <u></u></span></p>
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<p class="MsoNormal"><span><span style="color:#888888">-- </span><u></u><u></u></span></p>
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<p class="MsoNormal"><b><span style="color:#888888">César Augusto Ramírez Franco</span></b><span style="color:#888888"><br>
Laboratorio de Sistemas Complejos Naturales<br>
Escuela de Geociencias - Facultad de Ciencias<br>
Universidad Nacional de Colombia - Sede Medellín<br>
Teléfono: (57-4) 430 9369 - 300 459 6085</span><u></u><u></u></p>
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<p class="MsoNormal"><span style="color:#888888"><a href="http://labscn-unalmed.github.io/" target="_blank">http://labscn-unalmed.github.io/</a><u></u><u></u></span></p>
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<p class="MsoNormal"><br>
<br clear="all">
<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">-- <u></u><u></u></p>
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<p class="MsoNormal">Prof. Carlos Henrique Grohmann<br>
Institute of Energy and Environment - Univ. of São Paulo, Brazil<u></u><u></u></p>
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<p class="MsoNormal">- Digital Terrain Analysis | GIS | Remote Sensing - <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal"><a href="http://carlosgrohmann.com/" target="_blank">http://carlosgrohmann.com</a><u></u><u></u></p>
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<p class="MsoNormal"><a href="http://orcid.org/0000-0001-5073-5572" target="_blank"><span style="font-size:10.0pt;font-family:"Arial","sans-serif";color:#1155cc;background:white">http://orcid.org/0000-0001-5073-5572</span></a><u></u><u></u></p>
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<p class="MsoNormal">________________<br>
Can’t stop the signal.<u></u><u></u></p>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature"><div dir="ltr">Prof. Carlos Henrique Grohmann<br>Institute of Energy and Environment - Univ. of São Paulo, Brazil<div>- Digital Terrain Analysis | GIS | Remote Sensing - </div><div><br></div><div><a href="http://carlosgrohmann.com/" target="_blank">http://carlosgrohmann.com</a></div><div><a href="http://orcid.org/0000-0001-5073-5572" style="font-size:13px;color:rgb(17,85,204);font-family:arial,sans-serif;background-color:rgb(255,255,255)" target="_blank">http://orcid.org/0000-0001-5073-5572</a><br><div>________________<br>Can’t stop the signal.</div></div></div></div>
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