Given a set of cartesian coordinates, this function determines which points belongs to the convex hull (or envelope) of the set.
A vector of x (or longitude) coordinates.
A vector of y (or latitude) coordinates.
A numerical vector of the same length as
0 indicates that the corresponding point is not part of the convex
hull of the set. Values
>0 indicates that the corresponding point is
part of the convex hull, and each value corresponds to the order of the
point along the convex hull polygon.