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<p>Hi,</p>
<p>Your method in QGIS is sound. Area is calculated using the wgs84
ellipsoid EPSG 7030. If you reproject to wgs84 zone 35N (I think
this is close), area goes from 249566957499.7546m2 to
249566957499.721m2 or a difference of 0.0336 m2. I don't think
densification would change things much.</p>
<p>My question is the following: You know that QGIS uses the WGS84
Ellipsoid. What Ellipsoid are using used in the other
software??? If you don't know, I would proceed until you figure
that out.<br>
</p>
<p>This is the currently used ellipsoid in the proj database in QGIS
3.16. You can get this by typing proj -le in the OSFeo4W Shell.</p>
<p><br>
MERIT a=6378137.0 rf=298.257 MERIT 1983<br>
SGS85 a=6378136.0 rf=298.257 Soviet Geodetic System
85<br>
GRS80 a=6378137.0 rf=298.257222101 GRS 1980(IUGG, 1980)<br>
IAU76 a=6378140.0 rf=298.257 IAU 1976<br>
airy a=6377563.396 rf=299.3249646 Airy 1830<br>
APL4.9 a=6378137.0 rf=298.25 Appl. Physics. 1965<br>
NWL9D a=6378145.0 rf=298.25 Naval Weapons Lab.,
1965<br>
mod_airy a=6377340.189 b=6356034.446 Modified Airy<br>
andrae a=6377104.43 rf=300.0 Andrae 1876 (Den.,
Iclnd.)<br>
danish a=6377019.2563 rf=300.0 Andrae 1876 (Denmark,
Iceland)<br>
aust_SA a=6378160.0 rf=298.25 Australian Natl &
S. Amer. 1969<br>
GRS67 a=6378160.0 rf=298.2471674270 GRS 67(IUGG 1967)<br>
GSK2011 a=6378136.5 rf=298.2564151 GSK-2011<br>
bessel a=6377397.155 rf=299.1528128 Bessel 1841<br>
bess_nam a=6377483.865 rf=299.1528128 Bessel 1841 (Namibia)<br>
clrk66 a=6378206.4 b=6356583.8 Clarke 1866<br>
clrk80 a=6378249.145 rf=293.4663 Clarke 1880 mod.<br>
clrk80ign a=6378249.2 rf=293.4660212936269 Clarke 1880 (IGN).<br>
CPM a=6375738.7 rf=334.29 Comm. des Poids et
Mesures 1799<br>
delmbr a=6376428. rf=311.5 Delambre 1810
(Belgium)<br>
engelis a=6378136.05 rf=298.2566 Engelis 1985<br>
evrst30 a=6377276.345 rf=300.8017 Everest 1830<br>
evrst48 a=6377304.063 rf=300.8017 Everest 1948<br>
evrst56 a=6377301.243 rf=300.8017 Everest 1956<br>
evrst69 a=6377295.664 rf=300.8017 Everest 1969<br>
evrstSS a=6377298.556 rf=300.8017 Everest (Sabah &
Sarawak)<br>
fschr60 a=6378166. rf=298.3 Fischer (Mercury
Datum) 1960<br>
fschr60m a=6378155. rf=298.3 Modified Fischer 1960<br>
fschr68 a=6378150. rf=298.3 Fischer 1968<br>
helmert a=6378200. rf=298.3 Helmert 1906<br>
hough a=6378270.0 rf=297. Hough<br>
intl a=6378388.0 rf=297. International 1909
(Hayford)<br>
krass a=6378245.0 rf=298.3 Krassovsky, 1942<br>
kaula a=6378163. rf=298.24 Kaula 1961<br>
lerch a=6378139. rf=298.257 Lerch 1979<br>
mprts a=6397300. rf=191. Maupertius 1738<br>
new_intl a=6378157.5 b=6356772.2 New International 1967<br>
plessis a=6376523. b=6355863. Plessis 1817 (France)<br>
PZ90 a=6378136.0 rf=298.25784 PZ-90<br>
SEasia a=6378155.0 b=6356773.3205 Southeast Asia<br>
walbeck a=6376896.0 b=6355834.8467 Walbeck<br>
WGS60 a=6378165.0 rf=298.3 WGS 60<br>
WGS66 a=6378145.0 rf=298.25 WGS 66<br>
WGS72 a=6378135.0 rf=298.26 WGS 72<br>
WGS84 a=6378137.0 rf=298.257223563 WGS 84<br>
sphere a=6370997.0 b=6370997.0 Normal Sphere
(r=6370997)<br>
</p>
<p>Nicolas<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 2021-01-05 11:43 a.m., Rahkonen
Jukka (MML) wrote:<br>
</div>
<blockquote type="cite"
cite="mid:2c93e9b0d32244d4822bd0a83805b8ae@maanmittauslaitos.fi">
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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">I wonder what method
QGIS is using when it computes ellipsoidal area with $area
function. I made a test with this EPSG:4326 polygon:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">POLYGON (( 20.13293641
59.95688345, 26.94617837 60.47397663, 29.74782155
62.56499443, 27.45254202 68.7065034, 23.75771765
68.24937206, 25.42698984 65.27444593, 21.51545237
63.10353609, 21.4056276 61.12318104, 19.41123592
60.40477513, 20.13293641 59.95688345 ))<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I checked that the CRS
of the project and layer were both EPSG:4326. The ellipsoid
in the Project-Settings was WGS 84 (EPSG:7030). The area
that $area returns is<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">249566957499.7546<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">As a references I used
PostGIS and web site
<a
href="https://geographiclib.sourceforge.io/scripts/geod-calc.html#area"
moz-do-not-send="true">https://geographiclib.sourceforge.io/scripts/geod-calc.html#area</a>.
With my test polygon they both give this result:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">251199344354.4308<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">PostGIS returns bigger
area. The difference is 0.654% so not huge but not
negligible either.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">A third test with
SpatiaLite 5.0 gives a result that is very close to PostGIS
(difference 0.001%).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">251195856999.549927<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">As a conclusion I have
decided to trust in PostGIS because it gives the same
results than the web site
<a
href="https://geographiclib.sourceforge.io/scripts/geod-calc.html"
moz-do-not-send="true">https://geographiclib.sourceforge.io/scripts/geod-calc.html</a>
that feels scientifically sound. However, I wonder if I have
used QGIS in a correct way or if there is anything I could
do for getting areas to match better with PostGIS for
example by densifying the long segments in my test polygon.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">For getting testing easy
I attach directly runnable SQL for PostGIS and Spatialite
(version 5.0 with RTTopo is needed) as well as a coordinate
list for the web app.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">PostGIS<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">select
st_area(st_geogfromtext('POLYGON ((<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641
59.95688345,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">26.94617837
60.47397663,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">29.74782155
62.56499443,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">27.45254202
68.70650340,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">23.75771765
68.24937206,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">25.42698984
65.27444593,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.51545237
63.10353609,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.40562760
61.12318104,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">19.41123592
60.40477513,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641
59.95688345))') , true);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Spatialite<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">select
st_area(st_geomfromtext('POLYGON ((<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641
59.95688345,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">26.94617837
60.47397663,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">29.74782155
62.56499443,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">27.45254202
68.70650340,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">23.75771765
68.24937206,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">25.42698984
65.27444593,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.51545237
63.10353609,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.40562760
61.12318104,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">19.41123592
60.40477513,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641
59.95688345))') ,true);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Web site (notice lat-lon
order)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">59.95688345 20.13293641
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">60.47397663 26.94617837
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">62.56499443 29.74782155
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">68.70650340 27.45254202
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">68.24937206 23.75771765
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">65.27444593 25.42698984
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">63.10353609 21.51545237
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">61.12318104 21.40562760
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">60.40477513 19.41123592
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">59.95688345 20.13293641<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">-Jukka Rahkonen- <o:p></o:p></span></p>
</div>
<br>
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</blockquote>
<pre class="moz-signature" cols="72">--
Nicolas Cadieux
<a class="moz-txt-link-freetext" href="https://gitlab.com/njacadieux">https://gitlab.com/njacadieux</a></pre>
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