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When grading an exam, I discovered that QGIS 3.2 and 3.4 produce different values on computing $area over polygons.

I experimented using $area and area($geometry) to check the differences between planar and ellipsoid computations. It seems that QGIS 2.18 computes planar areas as default for $area, whereas QGIS 3.4 gives ellipsoidal areas for $area (as explained at the help section of the function).

In QGIS 3.2, $area produces different values (quite higher) than those for $area in QGIS 3.4, whereas area($geometry) produces the same values in the two versions.

As context, my data is projected on EPSG:32717 (WGS84 UTM z17 S).

Any insights on what's going on? Is this a bug? I'd like to explain the students these caveats and to be aware when computing area-derived variables.

area values for different QGIS versions

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    Some of the ellipsoidal area calculations were refined in 3.4. the values should be more accurate on 3.4 vs 3.2
    – ndawson
    Nov 13, 2018 at 7:52
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    @ndawson...will this be addressed in the 3.4 changelog? It's kind of a critical differnce when you're assessing large hectare lots! Nov 13, 2018 at 9:00
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    @ndawson please post your comment as an answer so this thread can be marked as answered
    – underdark
    Feb 15, 2019 at 19:09

1 Answer 1

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As @ndawson noted in the comments above:

Some of the ellipsoidal area calculations were refined in 3.4. the values should be more accurate on 3.4 vs 3.2

You can see the source code changes at: https://github.com/qgis/QGIS/commit/297dbe0786d30b7b05462c8dac49b51f13175a19 and https://github.com/qgis/QGIS/commit/d85039363a552b901da81467f45925f183bd50a6

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