Yerushalmi Eruvin 4:7 ירושלמי עירובין ד׳:ז׳

MISHNAH: If somebody was on the road and afraid lest it get dark¹⁶Before he can reach the next town. but he knew of a tree or a wall and says, “my Sabbath rest shall be under it,” he did not say anything¹⁷The place must be defined to within 4 cubits to be valid.. “My Sabbath rest in its roots”, he walks from the place of his feet to the roots 2’000 cubits and from the roots to his house 2’000 cubits; it turns out that he may walk 4’000 cubits after nightfall.
If he does not know or is not conversant with practice and said, my Sabbath rest shall be at my place, his place empowers him 2’000 cubits in every direction, circular according to Rebbi Ḥanania ben Antigonos. But the Sages say square like a square table, that he gain the corners¹⁸In the first opinion the Sabbath domain is a circular disk of radius 2’000 cubits. In the second, authoritative, opinion it is a square of side length 4’000 cubits oriented strictly NS-EW. From the center along the diagonals in fact one may walk 2’000 √2 = 2828 cubits. Most importantly, this definition also applies to Sabbath domains of towns which always are rectangular domains oriented strictly NS-EW..

HALAKHAH: If one recognized and one did not recognize; the one who recognized may go with the one who did not recognize and the one who did not recognize may go with the one who recognized⁸¹This refers to Mishnah 8, where a traveller who remembers a landmark between his situation at sundown and the next village may declare this landmark as his Sabbath center. A stranger who walks with him may adopt the other’s Sabbath center without knowing the geography. Babli 50b..