Calculate polygon perimeters in R-CodePudding

I have a polygon shapefile in R and would like to create a new column with each polygon's areas and perimeters. I have the following code which successfully generates areas, but not perimeters:

data<-arc.select(data.path) %>%
  arc.data2sf() %>% #convert data to SF object
  mutate(District=sub("^0 ", "",SLDUST20)) #remove leading 0s to district column

data$Shape_Area=st_area(data) #calculate district areas
data$Shape_length=lwgeom::st_perimeter(data) #calculate district perimeters

This yields the following error message: Error in lwgeom::st_perimeter(SD_senate) : for perimeter of longlat geometry, cast to LINESTRING and use st_length

CodePudding user response：

Set up data:

> library(sf)
> example(st_read) # get the `nc` object

Replicate your error:

> st_perimeter(nc)
Error in st_perimeter(nc) : 
  for perimeter of longlat geometry, cast to LINESTRING and use st_length

Try suggestion:

> st_length(st_cast(nc,"LINESTRING"))
Error in st_cast.sfc(st_geometry(x), to, group_or_split = do_split) : 
  use smaller steps for st_cast; first cast to MULTILINESTRING or POLYGON?

Try suggestion suggested by the suggestion:

> st_length(st_cast(nc,"MULTILINESTRING"))
Units: [m]
  [1] 141627.32 119875.85 160458.17 301643.62 211793.62 160780.01 150430.19
  [8] 123169.65 141072.70 140583.19 134369.71 157789.72 170610.20 132133.88
 [15] 112628.81 216204.53 116256.22 194694.89 124620.43  99987.87 116904.13

Note that even though the object is in lat-long coordinates you get perimeters in meters.