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I am puzzled on why I get different outcomes on trying to calculate the area (m2) for multiple polygons in QGIS.

My polygons consist of tiny objects of few cm extent. When using $area (field calculator / geometry) for the smallest polygons I get 0.0000 while saga tool gives a value.

It seems saga shape indices area calculation is more robust, but is there a reason why $area (geometry) fails to produce an outcome and even in visually different polygons it gives similar values or even bigger values in smaller polygons?

The EPSG is set to 32631 31N (meters) so it shouldn't cause any trouble. I don't know what I am missing here.

$Area (as calculated by $area), area (add geometry column Project CRS), Area (saga shape indices tool)

enter image description here

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    How did you define the field that stores the area calculation?You should have a decimal field with a length and precision similar to the image
    – Gerardo Jimenez
    Commented Oct 10, 2017 at 17:47
  • First of all thank you for your answers. Geraldo, I also thought it might be a not enough decimal situation but even with (decimal, precision = 10 and output field length =20) I didn't see any difference on the output - still values were 0.000000 whereas saga had an output value (small -but a value) In the image is the $Area (calculated with $area), area (from add geometry column/Layer CRS), and last Area (saga shape indices tool). ( I cannot upload another image?)
    – Christina Kallimani
    Commented Oct 11, 2017 at 8:23
  • $Area area (add geometry) Area (saga)|| 0.004394531 0.004394531 0.00375457|| 0 0 0.000579564|| 0.000976563 0.000976563 0.000352778|| 0.000976563 0.000976563 0.000251985 0.005859375 0.005859375 0.007206758 0.010253906 0.010253906 0.009096642 0.007324219 0.007324219 0.008617871 0.002441406 0.002441406 0.002318258 0.002441406 0.002441406 0.00309941 0.003417969 0.003417969 0.003074211 0.003417969 0.003417969 0.002872624 0.010253906 0.010253906 0.009046245 0.008300781 0.008300781 0.008391085 0.003173828 0.003173828 0.004437862
    – Christina Kallimani
    Commented Oct 11, 2017 at 8:37

1 Answer 1

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Are you using an ellipsoid for distance/area measurements? (Check via Project -> Project Properties, General section)

If so, I'd suggest changing that to "None/Planimetric". The ellipsoidal calculations are designed for larger area measurements, and if your area measurements are only a couple of centimetres than it's likely that the calculations aren't suitable. By setting the ellipsoid to "None" you'll skip the ellipsoid calculations and just calculate area in your chosen CRS.

Note that if your measurements/project was in a geographic CRS (latitude/longitude), setting the ellipsoid to "None" is not a good move and will result in inaccurate measurements!

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  • Ndawson, the measurements- ellipsoid properties is not selected, is set as (None/Planimetric) so I assume is not using an ellipsoid.
    – Christina Kallimani
    Commented Oct 11, 2017 at 8:25
  • Any chance you could share your data? If so, can you open an issue on issues.qgis.org and attach it? I'll take a look then.
    – ndawson
    Commented Oct 11, 2017 at 11:48

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