[Qgis-user] relation between the value of $area and the CRS (lat/long coordinates)

[Carlos Cerdán]

Maybe it's a bug, but I've had a similar issue. Solution:  I changed CRS to
UTM (my case), closed QGIS, open again and calculate areas (maybe I did a
reboot also, I don't remember), but after that, areas were calculated
correctly.

That time I was using Ubuntu 12.04 - QGIS 2.0.1.

Good luck

Carlos



2014-03-26 5:52 GMT-05:00 Manuel Campagnolo <mlc at isa.ulisboa.pt>:

> Hi,
>
> I have a question about calculating areas for polygon layers when
> coordinates are geographic (e.g. CRS=WGS84). In the example below I used a
> map of the US  (a MultiPolygon of the 50 States)
>
> 1): The layer and project CRS were WGS84 (EPSG:4326) -> The expression
> $area in field calculator returned a small value (1116.7), which I believe
> is obtained from lat/long coordinates (as if the map projection was Plate
> Carré) ;
> 2): I did change the Project CRS to a projected CRS
> (USA_Contiguous_Albers_Equal_Area_Conic) and I used $area again to
> compute the area -> the result was identical;
> 3) I reset the Project CRS to WGS84 and I created a 3rd field using again
>  $area -> this time around, the result was 9'475'180'617'431.81 (m2) -- !!
> (it is the same Layer and Project CRS than 1).
>
> If I reproject my Layer to USA_Contiguous_Albers_Equal_Area_Conic and
> then compute $area the result is 9'264'554'959'451.9 as expected.
>
> I would like to understand was is the rationale for $area (and other
> Geometry methods) in relation to the Layer and Project CRS when the
> coordinates are lat/long.
>
> Thanks,
> Manuel
>
>
>
> --
> _________________
> Manuel Campagnolo
> http://math.isa.utl.pt/~mlc/
>
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