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I've calculated the area of polygons with QGIS by using ctr-i - geometry - $area. I've set it on real. The results seem fine, but I've noticed they differ from the area given when I use the i-function.

For example: polygon 4 gives an area of 7,662 km2 when using the i-function but when I Calculate it, it gives an area of 7682816.27364349.

Does anybody know what I'm doing wrong?

Layer CRS is Amersfoort / RD new EPSG:28992

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What software are you using and precisely how are calculating the two areas you are comparing? –  whuber Mar 20 '13 at 14:04
    
It's the same area only the info box gives a different result than the calculating tool. –  Tom Mar 20 '13 at 15:27
    
i'm using Lisboa 1.8.0 –  Tom Mar 20 '13 at 15:27
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2 Answers

Amersfoort/RD new is based on the Bessel 1841 ellipsoid, which is not quite exact the same as the WGS84 ellipsoid you are measuring with by default.

At least it is a projected CRS, so you don't come up with square degrees.

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I don't know which tool/language you are using, but I guess parameter -i is giving you integer area while default (without -i flag) is a double precision. So I will go without -i since its more precise.

Nothing is wrong, its just an arithmetic issue.

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In what sense can 7,662 be construed as an integer equivalent of 7,682.81627364349? They are apart by more than 20 units! –  whuber Mar 20 '13 at 15:29
    
True, so something is wrong. I've noticed that in the option menu the ellipsoid for calculating distances (if translated correctly) is set on WSG84. Only i can't change it into EPSG:28992. I don't know if that's the problem though. –  Tom Mar 20 '13 at 15:33
    
From the internet: [i]To do area calculation correctly always reproject to a projected CRS. (Right click on the layer, "Save As..." and choose a suitable CRS). The area calculations in an unprojected CRS (like WGS84) will be in square degrees, and furthermore will be wrong. QGIS does not calculate area on a sphere, only on a planar CRS, so any length or area calculations done in an unprojected CRS will be misleading. [/i] –  Tom Mar 20 '13 at 15:59
    
What is meant with a projected CRS? –  Tom Mar 20 '13 at 16:01
    
whuber: I saw that they are off by 20m. I focused on int and float, since the area is calculated in small parts and the errors can accumulate to be larger (~20m.) And the guy above says its a wgs84 <-> RD New error which is quite likely. –  Naresh Mar 21 '13 at 13:39
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