The Gemora says (Bava Basra 122a, 6 lines up) that there was a financial adjustment in the land allotted in Eretz Yisroel to allow for "Kerovah Verechoka".
Rashbam says that land closer to Yerushalayim was preferred for two reasons:
(i) since nearer to the Beis HaMikdash, (ii) further from Eretz HaAmim.
I had Kashes on both of these, and saw that you addressed my kashes on the first reason (Beis HaMikdash) in your Insights On The Daf (thank you), but what about the second reason - surely there were several shevotim with borders next to other nations, and there were other shevotim with no border to other nations.
Kol Tuv
Meir Eliezer Bergman
Manchester UK
The Rashbam appears to be assuming that Yerushayaim is in the center of Eretz Yisrael, as Tosfos writes in Rosh Hashanah 23b DH Kamah, and Rashi Sotah 45a DH Sharerech (see Sanhedrin 37a). Thus, the further from Yerushalayim, the closer one is to the nations outside of Eretz Yisrael.
The Pnei Yehoshua in Rosh Hashanah says that he could not locate the source for Tosfos' contention that Yerushalayim is the center of Eretz Yisrael. However, the Mitzpeh Eisan points out that the source of Tosfos in Midrash Tanchuma to Kedoshim, #10.
Although Yerushalayim is much nearer to the Eastern border (the Jordan), apparently the Midrash it taking the width of the Transjordan into account (see Mitzpeh Eisan there). This puts Yerushalayim in the middle of Eretz Yisrael, both N/S and E/W. When the Shevatim compensated each other for the distance "from Yerushalayim," their compensation was based on the (N/S) distance from their territory to the lattitude of Yerushalayim, and the (E/W) distance from their territory to the longitude of Yerushalayim.
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf