<div dir="ltr">Hi Bernie and Andrea,<div><br></div><div>thank you very much for your feedback.</div><div><br></div><div>Andrea, regarding your question, which QGIS version and which method we are using, this is the method we are using in our programming pipeline...</div><div><a href="https://pyproj4.github.io/pyproj/dev/_modules/pyproj/geod.html#Geod.polygon_area_perimeter">https://pyproj4.github.io/pyproj/dev/_modules/pyproj/geod.html#Geod.polygon_area_perimeter</a><br></div><div><br></div><div>I am using QGIS version 3.28 LTR.</div><div><br></div><div>Best, Gabor</div></div><div id="DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2"><br><table style="border-top:1px solid #d3d4de"><tr><td style="width:55px;padding-top:13px"><a href="https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail" target="_blank"><img src="https://s-install.avcdn.net/ipm/preview/icons/icon-envelope-tick-round-orange-animated-no-repeat-v1.gif" alt="" width="46" height="29" style="width: 46px; height: 29px;"></a></td><td style="width:470px;padding-top:12px;color:#41424e;font-size:13px;font-family:Arial,Helvetica,sans-serif;line-height:18px">Não contém vírus.<a href="https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail" target="_blank" style="color:#4453ea">www.avast.com</a></td></tr></table><a href="#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2" width="1" height="1"></a></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, 20 Feb 2024 at 13:28, Andrea Giudiceandrea via QGIS-User <<a href="mailto:qgis-user@lists.osgeo.org">qgis-user@lists.osgeo.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">> Gabor Szakacs gabor.szakacs at <a href="http://agreena.com" rel="noreferrer" target="_blank">agreena.com</a><br>
> Tue Feb 20 03:58:40 PST 2024<br>
<br>
> 1) is CRS 4326 suitable to make precise area calculations of agriculture<br>
field boundaries that vary from 2 to 100ha for the 2nd decimal of a <br>
hectare?.<br>
<br>
> The avg area difference of the sum of all fields was in the 9th <br>
decimal of a hectare between area calculation with CRS 4326 and area <br>
calculation with regional equal area CRS for each region.<br>
<br>
Hi Gabor,<br>
it depends on how you calculate the area: using geodesic calculation on <br>
the ellipsoid, or planimetric calculation. If you use geodesic <br>
calculation, the CRS doesn't matter: you will obtain the same value <br>
provided you are calculating the are on the same ellipsoid and obviously <br>
that the coordinates of the vertices of the polygons are the same on <br>
such ellipsoid.<br>
<br>
> It seems that QGIS and Python use formulas that correctly convert<br>
degrees into metric area calculation and minimize the effect of different<br>
latitudes. Any kind of insights would be greatly appreciated.<br>
<br>
Which expressions / formulas / methods have you used? Which version of <br>
QGIS have you used?<br>
<br>
AFAIK, geodesic area and length calculation on the ellipsoid are <br>
performed in QGIS, since version 3.20, using the PROJ library which in <br>
turns uses, like PostGIS, the routines coded in the geographiclib library.<br>
<br>
For reference, see:<br>
<br>
- Use the GeographicLib routines exposed through PROJ for all <br>
ellipsoidal calculations <a href="https://github.com/qgis/QGIS/pull/41726" rel="noreferrer" target="_blank">https://github.com/qgis/QGIS/pull/41726</a><br>
<br>
- C. F. F. Karney, Algorithms for geodesics, J. Geodesy 87(1), 43–55 <br>
(Jan. 2013) <a href="https://doi.org/10.1007/s00190-012-0578-z" rel="noreferrer" target="_blank">https://doi.org/10.1007/s00190-012-0578-z</a><br>
- Errata / Addenda <a href="https://geographiclib.sourceforge.io/geod-addenda.html" rel="noreferrer" target="_blank">https://geographiclib.sourceforge.io/geod-addenda.html</a><br>
<br>
- GeographicLib::PolygonAreaT< GeodType > Class <br>
<a href="https://geographiclib.sourceforge.io/C++/doc/classGeographicLib_1_1PolygonAreaT.html" rel="noreferrer" target="_blank">https://geographiclib.sourceforge.io/C++/doc/classGeographicLib_1_1PolygonAreaT.html</a><br>
<br>
<br>
Best regards.<br>
<br>
Andrea<br>
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</blockquote></div><br clear="all"><div><br></div><span class="gmail_signature_prefix">-- </span><br><div dir="ltr" class="gmail_signature"><div dir="ltr"><span></span><span></span><table cellpadding="0" cellspacing="0" border="0" style="color:rgb(0,0,0);font-family:"Google Sans",Roboto,RobotoDraft,Helvetica,Arial,sans-serif;font-size:medium;background:none;border:0px;margin:0px;padding:0px"><tbody><tr><td style="font-family:Helvetica,Arial,sans-serif;color:rgb(61,67,68);font-size:12px"><b><br><br></b></td></tr></tbody></table><span><p dir="ltr" style="line-height:1.56;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(34,34,34);background-color:transparent;font-weight:700;vertical-align:baseline">Dr. Gabor Szakacs</span><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(34,34,34);background-color:transparent;font-weight:700;vertical-align:baseline"><br></span><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(19,79,92);background-color:transparent;vertical-align:baseline">Geospatial Analyst in Regenerative Agriculture</span></p><p dir="ltr" style="line-height:1.56;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(34,34,34);background-color:transparent;vertical-align:baseline">+55 11 989319305</span></p><p dir="ltr" style="line-height:1.56;margin-top:0pt;margin-bottom:0pt"><span style="font-size:8pt;font-family:Arial,sans-serif;color:rgb(19,79,92);background-color:transparent;vertical-align:baseline"><a href="mailto:gabor.szakacs@agreena.com" target="_blank">gabor.szakacs@agreena.com</a></span></p><br><p dir="ltr" style="line-height:1.56;margin-top:0pt;margin-bottom:0pt"><a href="https://agreena.com/" target="_blank"><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(17,85,204);background-color:transparent;vertical-align:baseline"><span style="border:none;display:inline-block;overflow:hidden;width:131px;height:35px"><img src="https://lh3.googleusercontent.com/HAYhoxGSnZE-f86DrpwIRP9ZpFrgoV7bKPgx-VPQK6bhSmFn-Y4qBFbIlawLDySqrV6naScXxfq83VxZzOcNjqHfxvc3mU6_-aN6LF-5obWS7P7zWBSaCHZZnhXeq8f4MP6DDHvlItY-c0HZo45EBEk" width="131" height="35" style="margin-left: 0px; 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