On Tue, Oct 2, 2018 at 7:30 PM Charles Karney <char...@karney.com> wrote: > > Sorry, I'm coming late to this conversation. The area of the polygon > in the Baltic Sea posted by Kristian is > > 14737935340.098511 m^2 > > assuming the WGS84 ellipsoid. This is accurate to 1 square-mm and was > obtained with the MPFR-enabled version of GeographicLib's Planimeter. > > This is consistent with the result reported by the PROJ.4 > geod_polygonarea() and it agrees with the result I get with the regular > Planimeter utility. I don't why someone else is getting a different > result from Planimeter.
I can confirm this, the results of Planimeter and PROJ 5.2.0 are identical. Some more confusion: I created simple boxes for the test polygon, one a bit larger, one a bit smaller than the test polygon GRASS: 13,222.778 Planimeter: 13,221.965 geod_polygonarea(): 13,221.965 14.62569 55.36254 14.6256944444444 54.0786 17.1177 54.0786111111111 17.1177777777778 55.3625 GRASS: 22,950.510 Planimeter: 22,946.901 geod_polygonarea(): 22,946.901 GRASS native geodesic area is not too far off from GeographicLib/PROJ With the test polygon Planimeter/PROJ: 14,737.935 km^2 GRASS GIS: 14.718.098 km^2 GRASS is much farther off, and here smaller instead of larger than Planimeter/PROJ. I will have a look. Markus M > It's probably operator error -- but who knows? > > --Charles > > On 10/2/18 10:08 AM, Kristian Evers wrote: > > Sorry, I don’t know. It’s possible there’s a bug somewhere. > > > > Charles, do you have any insights here? > > > > /Kristian > > > > *Fra:*Markus Metz <markus.metz.gisw...@gmail.com> > > *Sendt:* 2. oktober 2018 15:01 > > *Til:* Kristian Evers <kr...@sdfe.dk> > > *Cc:* GRASS developers list <grass-dev@lists.osgeo.org>; Helmut > > Kudrnovsky <hel...@web.de> > > *Emne:* Re: [GRASS-dev] area calculations in several GIS > > > > > > > > On Tue, Oct 2, 2018 at 12:08 PM Kristian Evers <kr...@sdfe.dk > > <mailto:kr...@sdfe.dk>> wrote: > > > > > > > > Use GeographicLib/PROJ as the reference. > > > > Which one? The results of Planimeter from GeographicLib and > > geod_polygonarea() from PROJ are different. If in doubt, use Planimeter > > as reference? > > > > Markus M > > > > > I would say that this polygon is large, yes. If you want to get a > > better sense for what constitutes a large polygon in this sense, try > > creating a series of polygons of varying size (e.g. polygons of your > > house, your city, your county, your country and your continent) and > > calculate the area with both GRASS and GeographicLib. At some point the > > calculated areas should start to diverge significantly. I suspect it > > happens somewhere between county and country. > > > > > > > > > > > > > > /Kristian > > > > > > > > > > > > Fra: Markus Metz <markus.metz.gisw...@gmail.com > > <mailto:markus.metz.gisw...@gmail.com>> > > > Sendt: 2. oktober 2018 11:51 > > > Til: Kristian Evers <kr...@sdfe.dk <mailto:kr...@sdfe.dk>> > > > Cc: GRASS developers list <grass-dev@lists.osgeo.org > > <mailto:grass-dev@lists.osgeo.org>>; Helmut Kudrnovsky <hel...@web.de > > <mailto:hel...@web.de>> > > > Emne: Re: [GRASS-dev] area calculations in several GIS > > > > > > > > > > > > > > > > > > On Tue, Oct 2, 2018 at 8:47 AM Kristian Evers <kr...@sdfe.dk > > <mailto:kr...@sdfe.dk>> wrote: > > > > > > > > Markus, > > > > > > > > > > > > Thanks for extending the list. I do wonder why Planimeter gives > > different results than geod_polygonarea(). I’ll run that by Charles > > Karney when I get a chance. > > > > > > > > > > for completeness, I used geod_polygonarea() from proj-5.2.0 > > > > > > > > > > > > >> Regarding areas in projected space: UTM is not area true, laea is > > but is instead scewing angles. You can check that stuff by running proj > > in very verbose mode. > > > > > > > > > > [...] > > > > > > > > > > > > > > So yes, the LAEA is the better choice of the two but it is never > > going to represent the true area, especially for large polygons, and I > > would not advice using it as reference for ellipsoidal area calculations. > > > > > > > > > > > > The question is, what can be used as reference? And is the test > > polygon a "large" polygon, causing "large" deviations from the true area > > when measured in LAEA? > > > > > > > > > > > > Markus M > > > > > > > > > > > > > > /Kristian > > > > > > > > > > > > > > > > > > > > > > > > Fra: Markus Metz <markus.metz.gisw...@gmail.com > > <mailto:markus.metz.gisw...@gmail.com>> > > > > Sendt: 1. oktober 2018 23:22 > > > > Til: GRASS developers list <grass-dev@lists.osgeo.org > > <mailto:grass-dev@lists.osgeo.org>>; Kristian Evers <kr...@sdfe.dk > > <mailto:kr...@sdfe.dk>> > > > > Cc: Helmut Kudrnovsky <hel...@web.de <mailto:hel...@web.de>> > > > > Emne: Re: [GRASS-dev] area calculations in several GIS > > > > > > > > > > > > > > > > Updated list with area calculations for > > > > > > > > https://gist.github.com/kbevers/207b5bcb9be20e7554abe5f56742ec2c > > > > > > > > > > > > > > > > PROJ [1]: 14,737.935 km^2 > > > > Caris LOTS: 14,737 km^2 > > > > ArcMap: 14,727.446 km^2 > > > > MapInfo: 14,727.352 km^2 > > > > GeoMedia: 14,726.443 km^2 > > > > Planimeter: 14,722.522 km^2 > > > > GRASS GIS: 14.718.098 km^2 > > > > > > > > EU LAEA [2]: 14,718.098 km^2 > > > > UTM 33 N: 14,707.742 km^2 > > > > > > > > QGIS 3.2: 14,652.181 km^2 > > > > QGIS 2.8: 14,652.181 km^2 > > > > > > > > [1] geodesic.h:geod_polygonarea() > > > > [2] EPSG:3035 > > > > > > > > > > > > > > > > In this case, GRASS GIS provides the best match of geodesic area to > > metric area. > > > > > > > > > > > > > > > > @Kristian: are the metric area measurements in "EU LAEA" and "UTM > > 33 N" suitable as reference? > > > > > > > > > > > > > > > > Discussion started on > > > > > > > > > > http://osgeo-org.1560.x6.nabble.com/Re-Qgis-user-New-Features-in-Shape-Tools-3-2-0-td5378898.html > > > > > > > > > > > > > > > > Markus M > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Oct 1, 2018 at 2:56 PM Markus Metz > > <markus.metz.gisw...@gmail.com <mailto:markus.metz.gisw...@gmail.com>> > > wrote: > > > > > > > > > > > > > > > > On Tue, Sep 25, 2018 at 7:38 PM Helmut Kudrnovsky <hel...@web.de > > <mailto:hel...@web.de>> wrote: > > > > > > > > > > fyi see > > > > > > > https://lists.osgeo.org/pipermail/qgis-developer/2018-September/054644.html > > > > > > > > > > with GRASS mentioned > > > > > ------------------ > > > > > Kristian Evers: > > > > > > > > > > Right, here are the calculated areas as returned by a number of > > different > > > > > GIS applications and the planimeter app of GeographicLib for > > reference: > > > > > > > > > > Caris LOTS: 14.737 km^2 > > > > > ArcMap: 14.727,446 km^2 > > > > > MapInfo: 14.727,352 km^2 > > > > > GeoMedia: 14.726,443 km^2 > > > > > Planimeter: 14.722,522 km^2 > > > > > QGIS 3.2: 14.652,181 km^2 > > > > > QGIS 2.8: 14.652,181 km^2 > > > > > > > > > > > > > > > > adding to the confusion: > > > > > > > > > > > > > > > > I used the geographiclib API as included in PROJ 5.2.0 following > > the example for geod_polygonarea() in geodesic.h and get > > > > > > > > geographiclib: 14,737.935 km^2 > > > > > > > > quite different from > > > > > > > > Planimeter: 14,722.522 km^2 > > > > > > > > > > > > > > > > GRASS native gives 14,718.097679 > > > > > > > > as also reported by Helmut and Stefan > > > > > > > > > > > > > > > > Markus M > > > > > > > > > > > > > > > > > > The polygon that I have used to get the numbers above can be > > found here: > > > > > https://gist.github.com/kbevers/207b5bcb9be20e7554abe5f56742ec2c > > > > > > > > > > I am quite confident that GeographicLib delivers the most > > accurate result > > > > > (if you have doubts, this reference [0] should convince you). As > > can be seen > > > > > from the table above all but QGIS come fairly close. I expect > > some variation > > > > > in the results as these are numerical approximations, although I > > think QGIS > > > > > is too far of the mark. My suspicion is that the geodesic > > algorithm used by > > > > > QGIS (and apparently GRASS) is to blame here. > > > > > > > > > > /Kristian > > > > > > > > > > [0] https://arxiv.org/pdf/1102.1215.pdf > > > > > ----------------- > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > ----- > > > > > best regards > > > > > Helmut > > > > > -- > > > > > Sent from: > > http://osgeo-org.1560.x6.nabble.com/Grass-Dev-f3991897.html > > > > > _______________________________________________ > > > > > grass-dev mailing list > > > > > grass-dev@lists.osgeo.org <mailto:grass-dev@lists.osgeo.org> > > > > > https://lists.osgeo.org/mailman/listinfo/grass-dev > > > > -- > Charles Karney <char...@karney.com> > Princeton, NJ
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