shapefile - Understanding unit of area sizes calculated via rgeos::gArea?

I'm interested in finding the unit for the obtained area size of the polygons without refererring to the externally available documentation.

A suite of shapefiles is imported as follows:

tmpDir <- tempdir()

tmpFle <- tempfile(fileext = ".zip")
download.file(url = "https://borders.ukdataservice.ac.uk/ukborders/easy_download/prebuilt/shape/infuse_rgn_2011_clipped.zip",
              destfile = tmpFle,
              method = "wget")
unzip(zipfile = tmpFle, exdir = tmpDir)
shpsUK <- rgdal::readOGR(dsn = tmpDir,
                         layer = tools::file_path_sans_ext(dir(tmpDir)[3]))

Area sizes

oldScipen <- getOption("scipen")

options(scipen = 999)
format(summary(rgeos::gArea(shpsUK, byid = TRUE)),
       big.mark = ",")
options(scipen = oldScipen)

which produces:

            Min.          1st Qu.           Median             Mean          3rd Qu.             Max. 
" 1,573,530,067" "13,003,803,228" "15,411,419,165" "14,492,421,008" "19,085,426,561" "23,851,569,172"

Can I determine the units of the obtained figures without referring to external documentation that may be available on the sources shapefiles?

Answer

The shapefiles you downloaded have projection information included in the .prj file:

PROJCS["OSGB_1936_British_National_Grid",GEOGCS["GCS_OSGB 1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northing",-100000],UNIT["Meter",1]]

After loading in R, you can get a summary of spatial information which gives you that information and more:

>summary(shpsUK)

Object of class SpatialPolygonsDataFrame
Coordinates:
      min      max

x 82672.0 655604.7
y  5337.9 657534.1
Is projected: TRUE 
proj4string :
[+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +datum=OSGB36 +units=m +no_defs +ellps=airy
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894]
Data attributes:
           geo_label      geo_code
 East Midlands  :1   E12000001:1  
 East of England:1   E12000002:1  

 London         :1   E12000003:1  
 North East     :1   E12000004:1  
 North West     :1   E12000005:1  
 South East     :1   E12000006:1  
 (Other)        :3   (Other)  :3  

The units are in metres (+units=m).