No, I know all the requirements for depot access (I've been playing DF since the first 3d version).
I was just wondering if there was a way to predetermine the direction/border traders and such would come from.
But since you can't, don't worry about it.
In some maps I actually take the time to channel 3 of 4 map edges to have only 1 tile... the unremovable edge tile.
Only even then I am afraid that both the smaller caravans and the imigrants mostly arrive randomly.
The only place the traders and imigrants dont enter the map is on the other side of a non-passable river.
Once I even had imigrants ariving on the channeled sides edge with a total of 6 tiles between trees... and then getting an ambush only shortly after the first imigrant arrives on the map on those same 6 tiles.
Was fun to watch, but not so fun to lose all the imigrants.
It would however be nice to be allowed to select an entry point, or entry edge to better predict the side the visitors are comming from.