Given a set of cartesian coordinates, this function determines which points belongs to the convex hull (or envelope) of the set.

isChull(x, y)

x | A vector of x (or longitude) coordinates. |
---|---|

y | A vector of y (or latitude) coordinates. |

A numerical vector of the same length as `x`

and `y`

.
`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.

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