I am working with a shapefile that has many polygons.
How do I add one more field named "area_sqkm" and calculate area for each polygon in the shapefile?
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1What libraries do you have installed and what have you tried?– GISHumanCommented Jun 29, 2016 at 15:57
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5rgeos::gArea(x,byid=TRUE)– mdsumnerCommented Jun 29, 2016 at 17:21
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1Welcome to gis.stackexchange! Please note that a good question on this site is expected to show some degree of research on your part, i.e. what you have tried and - if applicable - code so far. For more info, you can check our faq.– underdarkCommented Jun 29, 2016 at 19:53
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1 Answer
You can do
library(raster)
x <- shapefile('file.shp')
crs(x)
x$area_sqkm <- area(x) / 1000000
Or with terra
library(terra)
x <- vect('file.shp')
x$area_sqkm <- expanse(x) / 1000000
That works for both an angular CRS (longitude/latitude) and for planar CRSs. It is generally best to use a longitude/latitude CRS to compute area a planar CRS can distort size with a lot (How much? It depends on the CRS and on the location and extent of your spatial data).
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1is
raster::area()with unprojected data more accurate thanrgeos::gArea()with projected data? Commented Apr 30, 2019 at 22:45 -
2It can be. Because projections distort. If the inter-node distance of the polygon is high, projection can distort the area further (see the
geospherevignette). Commented May 1, 2019 at 0:14 -
2For other readers: This returns a numeric vector equal to the number of polygons (it looked to me as if it might be just for one polygon at a time) Commented Aug 27, 2021 at 21:23