How to convert data.frame to a list of polygon?

Question

I have a data frame like following:

long   lat   group  
1.0    1.0   1  
1.0    2.0   1  
2.0    2.0   1  
5.0    5.0   2  
5.0    6.0   2
6.0    6.0   2

How can I convert it into a list of polygon objs?

Thanks!

Answer 1

Your sequences of points aren't closed, so in sf, you'd need to munge a bit to turn them into polygons. One option is to make a multipoint object and cast to polygon:

library(tidyverse)
library(sf)

df <- data.frame(long = c(1, 1, 2, 5, 5, 6), 
                 lat = c(1, 2, 2, 5, 6, 6), 
                 group = c(1L, 1L, 1L, 2L, 2L, 2L))

df_sf <- df %>% 
    group_by(group) %>% 
    summarise(geometry = st_sfc(st_cast(st_multipoint(cbind(long, lat)), 'POLYGON'))) %>% 
    st_sf()

df_sf
#> Simple feature collection with 2 features and 1 field
#> geometry type:  POLYGON
#> dimension:      XY
#> bbox:           xmin: 1 ymin: 1 xmax: 2 ymax: 2
#> epsg (SRID):    NA
#> proj4string:    NA
#> # A tibble: 2 x 2
#>   group               geometry
#>   <int>              <POLYGON>
#> 1     1 ((1 1, 1 2, 2 2, 1 1))
#> 2     2 ((5 5, 5 6, 6 6, 5 5))

ggplot(df_sf) + geom_sf()