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

[Suncus Etruscus]

 Thank you for you answer, Michael,
I am new to the use of R for spatial analyses.

I have always used QGIS for this kind of operations, but this time I need
to repeat the area calculation with thousands of polygons several times in
a loop, so I switched to R.

Actually, what my work needs is the most accurate measurement of such
areas. I am wondering if the best thing to do is to accept the results in R
and ignore those in QGIS.

Thanks,
Nick

2018-06-27 17:29 GMT+02:00 Michael Sumner <mdsumner using gmail.com>:

> 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
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