Their southern border started at the bay on the southern tip of the Salt Sea,
Cross References
Joshua 15:1Now the allotment for the clans of the tribe of Judah extended to the border of Edom, to the Wilderness of Zin at the extreme southern boundary:
Joshua 15:3proceeded south of the Ascent of Akrabbim, continued on to Zin, went over to the south of Kadesh-barnea, ran past Hezron up to Addar, and curved toward Karka.
Treasury of Scripture
And their south border was from the shore of the salt sea, from the bay that looks southward:
the salt sea
Joshua 3:16
That the waters which came down from above stood and rose up upon an heap very far from the city Adam, that is beside Zaretan: and those that came down toward the sea of the plain, even the salt sea, failed, and were cut off: and the people passed over right against Jericho.
Genesis 14:3
All these were joined together in the vale of Siddim, which is the salt sea.
Numbers 34:3
Then your south quarter shall be from the wilderness of Zin along by the coast of Edom, and your south border shall be the outmost coast of the salt sea eastward:
bay [heb] tongue
Isaiah 11:15
And the LORD shall utterly destroy the tongue of the Egyptian sea; and with his mighty wind shall he shake his hand over the river, and shall smite it in the seven streams, and make men go over dryshod.