The lot for the tribe of the children of Judah according to their families was to the border of Edom, even to the wilderness of Zin southward, at the uttermost part of the south.
Ancient Near Easterners used natural landmarks (rivers, mountains, deserts, towns, etc.) to construct borders as well as artificial boundaries that they made by drawing lines between sites. Virtually all nations have used these methods, and they are still common today.