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Normally if you measure the distance of line and polygon in Cartesian is smaller compare to Ellipsoidal in QGIS.

However, when I am measuring the distance Cartesian system gives greater value compare to Ellipsoidal as you can see in the attachment.

Why is that?

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PolyGeo
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Hasan
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    This is because you are using EPSG:3857 (Web Mercator) and it is not possible to get an accurate distance from Web Mercator. You need to use a better projection, such as UTM with a correct zone in which your data is located to get better distance measurements. There are many questions talked about the inaccuracy of EPSG:3857 in measuring distances: https://gis.stackexchange.com/a/242547/63384 and Getting accurate distance measurements using EPSG:3857 in QGIS. – ahmadhanb Oct 19 '22 at 07:27
  • Are you sure that "Normally if you measure the distance of line and polygon in Cartesian is smaller compare to Ellipsoidal in QGIS" is Always true for all projection ? – J.R Oct 19 '22 at 10:23
  • @-J.R https://gis.stackexchange.com/questions/238759/why-is-ellipsoidal-distance-greater-than-cartesian-distance#:~:text=Cartesian%20distance%20is%20as%20measured,whose%20corners%20touch%20the%20circle. – Hasan Oct 19 '22 at 10:47
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    Do you have an understanding what the difference is between cartesian vs. ellipsoidal distance measurements? Very simply sayed: if at all, ellipsoidal distances are "real-world" distances, whereas cartesian distances (often) do not have any real-world meaning. They are just distances in the projected canvas, thus often with huge distortions. – Babel Oct 19 '22 at 12:00

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