They are the same only for geographical entities, because the construction also have other interpretation and those can be different for some cases.

• Faire un tour de can also mean “have a ride on” (a vehicle, a carousel, a rollercoaster…).
• Faire le tour de has the literal meaning “make one’s way in circle around”.

As such faire un tour de carousel and faire le tour du carousel mean different thing: the former means riding on it, the later walking around it.

Additionally, you may note in that example a slight different in syntax, in that “faire le tour de” usually requires a definite complements, whereas faire un tour de doesn’t.