The Mishna, Eruvin 4:7, says that someone approaching his home before Shabas can designate a spot between his home and his current location as his Shabas location, and he gets a 2000-ama radius around that spot, so he can go home:
One who was coming on the road and it was getting dark for him, and he recognized a tree or fence and said "My spending Shabas is under it" — he said nothing [because the area under the tree or fence is too large to be designated as a location for these purposes]. "My spending Shabas is at the base of [that tree]" — he [may] walk from the place his feet are in until its base, [up to] 2000 amos, and from its base until his home, [up to another] 2000 amos. It turns out he walks, after it gets dark, 4000 amos.
Then 4:8 continues:
If he doesn't recognize [such a landmark], or is not an expert in halacha [to know that he can use the above rule], and he said [instead] "My spending Shabas is in my location", he has acquired his location for himself, 2000 amos in every direction.
Seemingly, then, this designation from afar must be at a tree or other landmark. "If he doesn't recognize" one, he can't do it. Why is that? Why can't he say "My spending Shabas is at the point 2000 amos in that direction" (pointing, or saying "east")? Why isn't that a good enough designation?