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This may be a suitable approach, not specifically the ellipse axis approach, but:</div>
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Vector -> Geometry tools -> Centroid</div>
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may do what you want, depending on exactly how QGIS calculates the centroid...</div>
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<div class="PlainText">Brent Wood<br>
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Principal Technician, Fisheries<br>
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DDI: +64 (4) 3860529</div>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Qgis-user <qgis-user-bounces@lists.osgeo.org> on behalf of Azzurra Lentini <azzurralentini@gmail.com><br>
<b>Sent:</b> Wednesday, January 13, 2021 08:04<br>
<b>To:</b> qgis-user <qgis-user@lists.osgeo.org><br>
<b>Subject:</b> [Qgis-user] how to find center of polygons and/or major and minor diameters</font>
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<div class="x_gmail_default" style="font-family:tahoma,sans-serif"><font style="background-color:rgb(255,255,255)" color="#000000">Hi to all, any suggestion on how to quickly draw <span style="font-family:inherit; white-space:pre-wrap">the minor and major diameters
of different polygons? I have a shapefile of polygons: some of the polygons are ellipses and others are similar to ellipses but not regular.
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<div class="x_gmail_default" style="font-family:tahoma,sans-serif"><font style="background-color:rgb(255,255,255)" color="#000000"><span style="font-family:inherit; white-space:pre-wrap">My aim is to find the center of these polygons (In this aim I was thinking
to draw the 2 diameters and to use the intersection of these diameters as a center point of the polygon)</span></font></div>
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<div class="x_gmail_default" style="font-family:tahoma,sans-serif"><font style="background-color:rgb(255,255,255)" color="#000000"><span style="font-family:inherit; white-space:pre-wrap">Thanks, Azzurra
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<div dir="ltr">Lecturer GIS University "Roma Tre"<br>
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<div dir="ltr">Consultant Environmental Risk Prevention and Hydrogeology<br>
AZZURRA LENTINI </div>
<div dir="ltr">++++++++++++++++++++++++++<br>
Italy Mobile Tel.: **(39) 338 24 40 676<br>
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SKYPE azzurrahydro<br>
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<b><u><span style="font-size:12pt; font-family:Arial,sans-serif; color:blue"><a href="mailto:azzurralentini@gmail.com" target="_blank" style="color:rgb(17,85,204)">azzurralentini@gmail.com</a></span></u></b></p>
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<b><u><span style="font-size:12pt; font-family:Arial,sans-serif; color:blue"><a href="mailto:azzurra.lentini@uniroma3.it" target="_blank" style="color:rgb(17,85,204)">azzurra.lentini@uniroma3.it</a> </span></u></b><b><u><span style="font-size:12pt; font-family:Arial,sans-serif; color:blue"> </span></u></b><br>
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<div dir="ltr">++++++++++++++++++++++++++<br>
*Par respect pour l'environnement,*<br>
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*n'imprimez ce mail qu'en cas d'absolue nécessité*</div>
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<span>Brent Wood<br>
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