Convert sequence of longitude and latitude to polygon via sf in R

Question

I have five longitude and latitude that form a shape like this.

df <- c(order=1:5,
        lon=c(119.4,119.4,119.4,119.5,119.5), 
        lat=c(-5.192,-5.         


        

Answer 1

The equivalent as @Yo B. answer but with sf

library(sf)
df <- data.frame(lon=c(119.4,119.4,119.4,119.5,119.5), 
                 lat=c(-5.192,-5.192,-5.187,-5.187,-5.191))

# You need first to close your polygon 
# (first and last points must be identical)
df <- rbind(df, df[1,])

poly <- st_sf(st_sfc(st_polygon(list(as.matrix(df)))), crs = 4326)
poly

## Simple feature collection with 1 feature and 0 fields
## geometry type:  POLYGON
## dimension:      XY
## bbox:           xmin: 119.4 ymin: -5.192 xmax: 119.5 ymax: -5.187
## epsg (SRID):    4326
## proj4string:    +proj=longlat +datum=WGS84 +no_defs
##   st_sfc.st_polygon.list.as.matrix.df....
## 1          POLYGON ((119.4 -5.192, 119...

edit to answer a question in the comments

See the main sf vignette for a clear and detailed explanation of sf, sfc and sfg objects summarized as :

The three classes used to represent simple features are:

• sf, the table (data.frame) with feature attributes and feature geometries, which contains
• sfc, the list-column with the geometries for each feature (record), which is composed of
• sfg, the feature geometry of an individual simple feature.

The st_sfc function builds only the geometry column (which is a list of polygons - here with only one polygon). The "c" in sfc stands for "column". The function st_sf builds a full sf object (which has also a data.frame class) which is a data frame with a geometry column. In the given example there is no data attached to the polygon (no attributes). You can attach data by building a data.frame :

poly <- st_sf(data.frame(landuse = "Forest", 
                         size = 23 , 
                         st_sfc(st_polygon(list(as.matrix(df))))), 
              crs = 4326)
poly
## ## Simple feature collection with 1 feature and 2 fields
## geometry type:  POLYGON
## dimension:      XYZ
## bbox:           xmin: 1 ymin: 119.4 xmax: 5 ymax: 119.5
## epsg (SRID):    4326
## proj4string:    +proj=longlat +datum=WGS84 +no_defs
## landuse size                       geometry
## 1  Forest   23 POLYGON Z ((1 119.4 -5.192,...

You can then extract each of these elemnts form the spatial object and check their class :

Full sf object : a data.frame with a sfc geometry column

class(poly)
## "sf"         "data.frame"

Third column extracted as a list : sfc object

class(poly[[3]])
## "sfc_POLYGON" "sfc"    

First element of the geometry column : an sfg polygon object

class(poly[[3]][[1]])
## "XY"      "POLYGON" "sfg"