[GRASS-dev] Fw: Re: SV: area calculations in several GIS

[Helmut Kudrnovsky]

for the record in the ML see below
 
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Gesendet: Dienstag, 02. Oktober 2018 um 19:30 Uhr
Von: "Charles Karney" <>
An: "Kristian Evers" <>, "Markus Metz" <>
Cc: "GRASS developers list" <grass-dev at lists.osgeo.org>, "Helmut Kudrnovsky" >
Betreff: Re: SV: [GRASS-dev] area calculations in several GIS

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. 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 
> *Sendt:* 2. oktober 2018 15:01
> *Til:* Kristian Evers
> *Cc:* GRASS developers list <grass-dev at lists.osgeo.org>; Helmut
> Kudrnovsky 
> *Emne:* Re: [GRASS-dev] area calculations in several GIS
>
>
>
> On Tue, Oct 2, 2018 at 12:08 PM Kristian Evers 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
> >
--
Charles Karney <charles at karney.com>
Princeton, NJ