title: Boundary points from a set of coordinates
body: <p>I have a set of lat,long points, and from this points I'd like to extract the points that form the boundaries, I've used convexhull, but for my purpouse is not enough as convehull just returns the most distant points that form the polygon where all the points fit, I need ALL the points that form the peremiter, something like the image I've attached.  What could I do? Is there some kind of package ready to use instead of implement any spatial algorithm?
  Thanks
   <a href="https://i.stack.imgur.com/SqSlN.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/SqSlN.jpg" alt="enter image description here"></a> </p>