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I'm calculating the area of ​​some polygons and realized that the values ​​are coming out differently when I do the calculations in ArcGIS and QGIS. The data is in Sirgas 2000 and projected for the South America Albers Equal Area Conic.

Also, I can't control the number of decimal places for this field in ArcGIS, even by changing the Precision and Scale values.

Can anyone tell me what I might be doing wrong or is it normal?

I'm using field calculator in both cases. In ArcGIS I've selected the spatial reference of the file plus the square kilometers option. In QGIS I've used "$area / 100000". The datum is "Sirgas 2000" and I've done the projection previously in ArcGIS (Project - Data Management)

enter image description here

The areas in the picture are in square kilometers (km²).

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    Can you explain precisely how you calculated the areas in both packages; the calculation for polygon area should be the same in both but might be slightly influenced if the area is being calculated in a different spatial reference. – Michael Stimson Aug 1 '18 at 0:50
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    My guess would be the Ellipsoid used for the area calculations is different in each software package. The default for the projection you have used in QGIS is Austrailian Natl & S. American 1969. I am not sure what the default is in ArcMap. – AWGIS Aug 1 '18 at 7:42
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    Please Edit the question in response to requests for clarification. It's not fair to those who would answer to need to mine the comments for critical information. – Vince Aug 1 '18 at 11:19
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    @AWGIS were right. For me, the default Ellipsoid in QGIS for this project were WGS84. Switching to None / Planimetric gave me the same results of ArcGIS (and even PostGIS, which I've used to test too). Thanks everyone! – Rodrigo Mota Aug 1 '18 at 13:34
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    Oi Johnny! A diferença está no Elipsóide usado como base no cálculo. Por padrão, o QGIS utiliza o WGS84. Alterando para 'None / Planimetric' encontrei os mesmos valores que no ArcGIS e PostGIS. Para alterar essa configuração vá em Propriedades do Projeto > Geral > Medições > Elipsóide. – Rodrigo Mota Nov 12 '18 at 18:44
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My guess would be that the Ellipsoid used for the area calculations is different in each software package.

The default for the projection you have used in QGIS is Austrailian Natl & S. American 1969. I am not sure what the default is in ArcMap.

If they are different then this will explain the differences you are getting.

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    It is worth noting that to check which ellipsoid you're applying to your project on QGIS, you should go to Project (on the top left) > proprierties and select the general tab. There, you will have a subsection informing which ellipsoid is being used. Don't know the path on Arcmap. Maybe the answer could be edited to include such information. – Eric Lino May 7 '19 at 18:05
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From the Help page of ArcGIS 10.3 "About calculating geometry":

"Geometry calculations in ArcGIS are planimetric - in other words, they take place in projected space, not spherical or geodesic space".

From the other hand, the $area in QGIS gives you elipsoidal areas if an elipsoid is set up for the project.

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