Add km to units sf knows about

[karldw]

Currently the CRS parsing code in crs_parameters doesn't know about projections in kilometers. Here's a quick demo, contrasting with a projection in meters.

library(sf)
km_crs <- "+init=epsg:5070 +units=km"
nc <- st_read(system.file("shape/nc.shp", package="sf")) 
nc_km <- st_transform(nc, "+init=epsg:5070 +units=km")
nc_m <- st_transform(nc, "+init=epsg:5070 +units=m")

sf:::crs_parameters(st_crs(nc_km))  # fails
sf:::crs_parameters(st_crs(nc_m))   # works

# spelling of "kilometre"
sf:::CPL_crs_parameters("+init=epsg:5070 +units=km")[[3]] == "kilometre"  # TRUE

[edzer]

Thanks, this revealed a larger problem: that sf assumed so far that a single epsg code always refers to a unique proj4string. Your example illustrates the opposite.

[edzer]

... and there are more units to convert:

cs2cs -lu
          km 1000.                Kilometer
           m 1.                   Meter
          dm 1/10                 Decimeter
          cm 1/100                Centimeter
          mm 1/1000               Millimeter
         kmi 1852.0               International Nautical Mile
          in 0.0254               International Inch
          ft 0.3048               International Foot
          yd 0.9144               International Yard
          mi 1609.344             International Statute Mile
        fath 1.8288               International Fathom
          ch 20.1168              International Chain
        link 0.201168             International Link
       us-in 1./39.37             U.S. Surveyor's Inch
       us-ft 0.304800609601219    U.S. Surveyor's Foot
       us-yd 0.914401828803658    U.S. Surveyor's Yard
       us-ch 20.11684023368047    U.S. Surveyor's Chain
       us-mi 1609.347218694437    U.S. Surveyor's Statute Mile
      ind-yd 0.91439523           Indian Yard
      ind-ft 0.30479841           Indian Foot
      ind-ch 20.11669506          Indian Chain