[R-sig-Geo] Difference in area calculation between QGIS and R

[Suncus Etruscus]

Dear List,
I am trying to use R ("sp" and "raster" packages) to calculate the area of
several polygons (CRS of the shapefile EPSG: 4326  - WGS84).

I used this line of code:

shapefile_name$area_km2 <- area(shapefile_name)/1000000

However, when I used QGIS to calculate the area of the same polygons
(through the "$area" function), I found there was a slight difference (in
every polygon).

For example, for a polygon of about 30 000 km2, the area calculated in R
was 50 km2 smaller.

What could be the cause?

Thanks in advance,
N.

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