The 9th-level Sor/Wiz spell teleportation circle [conj] (PH 293) says that
You create a circle on the floor or other horizontal surface that teleports, as greater teleport, any creature who stands on it to a designated spot.
But it's the DM's decision both how one goes about designating and what, exactly, is a spot. Here are some choices:
- Appearance: Designate a spot by describing a location. The greater, more unique, and specific the details, the more unlikely a false destination.
- Coordinates: Designate a spot by assigning it a latitude and longitude. Thus spots that move can't be designated.
- Crosshairs: Designate a spot by selecting a crosshairs; spots the caster's never seen can still be designated. Thus only locations large enough to have their own maps (e.g. a ship, a flying fortress) will have spots that can be designated.
- Name: Designate a spot by using its name, and employing the spell teleportation circle to travel to the named spot takes the creature to the location. (I suggest the location's most commonly used entrance.)
- Relative Location: Designate the spot by its relationship to something else (e.g. creatures, landmarks, objects, terrain features).
Also keep in mind that while the caster can't arrive off target using the 7th-level Sor/Wiz spell greater teleport (PH 293) (hence neither with the spell teleportation circle), he must still roll on the chart for the 5th-level spell teleport [conj] (PH 292-3) when the effect's used, and the possibility of a false destination or outright failure still exists if using the spell teleportation circle to teleport blind. (Despite the spell teleportation circle failing if its caster lacks familiarity with the destination, how one can become just familiar (rather than very familiar as described in the spell teleport) with a destination is never made clear.)