C. F. Keil and Franz Delitzsch on Joshua 18:15-19

Jos 18:15-19

“As for the southern boundary from the end of Kirjath-jearim onwards, the (southern) boundary went out on the west (i.e., it started from the west), and went out (terminated) at the fountain of the water of Nephtoah.” Consequently it coincided with the northern boundary of Judah, as described in Jos 15:5-9, except that it is given there from east to west, and here from west to east (see at Jos 15:5-9). In the construction haגּבוּל תּוצאותיו, the noun הגּבוּל is in apposition to the suffix: the outgoings of it, namely of the border (see Ewald, §291, b.).