On 06/27/2018 05:29 PM, Michael Sumner wrote: > Raster uses a (discretized) cosine-of-latitude approximati (popular amongst > longlat map makers).
As far as I can tell, raster branches into the libgeographic code (copied into the package sources, although Karney is not mentioned as one of the package copyright holders), using close to exact geodesic / ellipsoidal computation. > > 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 >> -- Edzer Pebesma Institute for Geoinformatics Heisenbergstrasse 2, 48151 Muenster, Germany Phone: +49 251 8333081 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo