Then one way to fill in the missing requirement would be a "local" convex hull, where the definition of "local" can vary. My first idea would be to define a radius length that is a bit longer than the maximum "distance to closest neighbor point" (or just set by some person), pick an extreme point (leftmost, for example), draw the radius starting at that point and pointing "away" from the other points (to the left, using the same example), then sweep it clockwise from there until it hits another point. Then draw the radius from this new point back toward the previous point and then spin the radius clockwise again to find the next point. Continue until you get back to the starting point, at which time your sequence of points will have defined a closed polygonal border at least around some (at least two) of the points (if there are points left over, repeat the process for them).