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

[Manuel Campagnolo]

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/