sf package point in polygon

Question

I'm trying to create a simple polygon in sf and select only points within that polygon. What am I doing wrong here?

library(sf)
library(concaveman)
library(ggplot2)

foo.df <- data.frame("long"=c(136,137,137,136),"lat"=c(36,36,37,37))
foo.sf <- st_as_sf(foo.df, coords = c("long","lat"))
poly <- concaveman(foo.sf) ## in case points are out of order
point.df <- data.frame("long"=c(136.2,136.5,137.5),"lat"=c(36.5,36.5,36.5))
point.sf <- st_as_sf(point.df, coords = c("long","lat"))

good_points <- st_join(point.sf,poly,join=st_within)

The st_join function doesn't seem to do anything

ggplot() + 
  geom_sf(data = poly) +
  geom_sf(data= good_points)

The problem isn't with the concaveman package

good_points <- st_join(point.sf,foo.sf,join=st_within)

ggplot() + 
  geom_sf(data = poly) +
  geom_sf(data= good_points)

And this attempt to create a polygon throws an error

another_poly <- st_polygon(list(as.matrix(foo.df)))
good_points <- st_join(point.sf,another_poly,join=st_within)

What am I missing?

Answer 1

You can use st_intersects() and it will let you know which points are within the polygon.

Add st_intersects(poly, goodpoints)[[1]] and it will return [1] 1 2.

If you add coordinate points 136.1, 36.1 to your list it will return [1] 1 2 4.