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When I have found geometry attributes in the past, I've found my distances are often out by quite a lot unless I use an ellipsoidal CRS to calculate them. Why is this the case?

Should I be using ellipsoidal or is there something wrong with what I'm doing. What is the advantage/disadvantage of using 'ellipsoidal' on QGIS to calculate geometry attributes?

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    What distances are you talking about? Which CRS are you using? – Erik Aug 27 at 9:15
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    Again: What distances are you talking about? Which CRS are you using? – Erik Aug 27 at 9:17
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    @Alex - Ellipsoidal calculations are used for long distances as it takes into account the curvature of the Earth; planimetric projections calculates the distance as if it was on a flat plane which can be fine for short local distances. There's plenty of information regarding this on GIS:SE and online. – Joseph Aug 27 at 9:37
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    You're comparing geodetic area calculation to Cartesian area in Web Mercator at high latitudes? Web Mercator should never be trusted for distance. – Vince Aug 27 at 10:53
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    Quite the opposite. Area calculation in Web Mercator is useless (the poles do not have infinite area). If you're going to validate area, it should be with an Equal Area projection. – Vince Aug 27 at 12:24

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