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I would like to calculate the surface of a group of polygons in QGIS. It was not me to create the shapefile, when I add it to the project and I check its properties, it results that the geometry and coordinate reference system is EPSG:32633 (layer properties/source).

I went to project/properties and in general/unit for area measurement to set “hectares”. At this point I have two doubts: 1) Which should be the right option to select in Ellipsoid (project properties/general) and 2) is it sufficient to use $area to calculate the surface?

I would like to add that these polygons have the dimension of census tracks, but I can aggregate them in larger units (Dissolve by..). Does it make a difference if I apply $area to small surfaces or the biggest one? Which one would provide the most accurate results?

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    Please Edit the question to specify the software you are using.
    – Vince
    Commented Mar 12, 2022 at 23:14
  • It is QGIS, sorry.
    – Geo_it
    Commented Mar 13, 2022 at 17:26

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Assuming you work in Slovenia, I would recommend using the Slovenia Grid EPSG:3794, this gives you the least distortion. Of course if you work on a very detailed scale like 1:10 000 your area calculations using $area are also more accurate. Please contact the national survey if the results are going to be used for legal business.

Considering you are working in Italy, that makes it a bit more complicated. The central meridian for EPSG:32633 is 15E, so west of 12E your area calculation is not correct anymore, there you must use EPSG:32632.

For Italy it would be recommended to work with the Monte Mario Datum and calculate areas directly on the International 1924 ellipsoid.

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  • No, it is Italy.
    – Geo_it
    Commented Mar 13, 2022 at 17:09
  • The most detailed geographical unit is the census track (or better - portions of census track) but then I'll need to conduct an analysis on a wider scale. That is why I don't know if it is better to 1) calculate the surface of the census track and then sum up the areas corresponding to my unit of analysis 2) dissolve and calculate the surfaces for these wider areas.
    – Geo_it
    Commented Mar 13, 2022 at 17:11

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