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 <mdsum...@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.etruscu...@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. >> >> [[alternative HTML version deleted]] >> >> _______________________________________________ >> R-sig-Geo mailing list >> R-sig-Geo@r-project.org >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo >> > -- > Dr. Michael Sumner > Software and Database Engineer > Australian Antarctic Division > 203 Channel Highway > <https://maps.google.com/?q=203+Channel+Highway+Kingston+Tasmania+7050+Australia&entry=gmail&source=g> > Kingston Tasmania 7050 Australia > <https://maps.google.com/?q=203+Channel+Highway+Kingston+Tasmania+7050+Australia&entry=gmail&source=g> > > [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo