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,-5.192,-5.187,-5.187,-5.191))
How could I easily convert them into an sf polygon data frame using sf
package like this?
## 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
## geometry
## 1 POLYGON ((119.4 ...
I saw that this question is coming up in search results, so I thought I'd provide a more flexible method of creating polygons in sf
from a series of lat
and lon
coordinates.
st_as_sf
has an argument coords
that will take points given as coordinate columns in a data frame and convert those columns to sf
POINT
geometries. Then, because sf
works well with dplyr
, we can st_combine
the points into a MULTIPOINT
and st_cast
to convert to POLYGON
. Compared to "manual" construction with st_polygon
, this has the advantage that we don't have to think so carefully about closing the ring or about the right level of nested lists to pass to the constructor, and that if we have more than one polygon in a set of coordinates we can use group_by
to create all the polygons at once.
N.B. Technically you can do this with do_union=FALSE
inside of summarise
, but I think that this syntax is a bit clearer and more similar to normal summarise
.
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)
)
library(tidyverse)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, proj.4 4.9.3
polygon <- df %>%
st_as_sf(coords = c("lon", "lat"), crs = 4326) %>%
summarise(geometry = st_combine(geometry)) %>%
st_cast("POLYGON")
polygon
#> 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
#> geometry
#> 1 POLYGON ((119.4 -5.192, 119...
plot(polygon)
Created on 2018-10-05 by the reprex package (v0.2.0).
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"
library(sfheaders)
on CRAN from 20191004 can take a data.frame and convert it to sf
objects
library(sf)
library(sfheaders)
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)
)
sfheaders::sf_polygon(
obj = df
)
## given only two columns of data are in df there's no need to specify lon & lat arguments
# Simple feature collection with 1 feature and 1 field
# geometry type: POLYGON
# dimension: XY
# bbox: xmin: 119.4 ymin: -5.192 xmax: 119.5 ymax: -5.187
# epsg (SRID): NA
# proj4string:
# id geometry
# 1 1 POLYGON ((119.4 -5.192, 119...
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