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

[Michael Sumner]

Raster uses a (discretized) cosine-of-latitude approximati (popular amongst
longlat map makers).

QGIS uses a project to local equal area projection method or maybe some
other approach.

There's lots and f options, all that matters is what your work needs.

Cheers, Mike

On Wed, 27 Jun 2018, 22:14 Suncus Etruscus, <suncus.etruscus.i using gmail.com>
wrote:

> 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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>
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-- 
Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia

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