The gemara (Zevachim 62) tells us that nevi'im came to Eretz Yisrael with Ezra and testified as to the correct location of the mizbeyach. Why did it take the word of a navi to figure out where the mizbeyach should be built? Do we need a navi before we can build a mizbeyach? The Maharat"z Chiyus explains that the first Beis haMikdash was razed to the point that not even the foundations of what originally stood remained. There was no evidence left that would indicate where walls were, where the mizbeyach had been, etc. Therefore, it required the help of a navi to figure out where to place things. Today, since we have archeological evidence and we have remnants of the Mikdash still left standing, we can simply take out a tape measure and figure out based on the dimensions given in Mes. Midos exactly where things go. We can theoretically rebuild the mizbeyach even without a navi.
The Brisker Rav (on Bechoros 17) disagrees. The Rambam writes (Beis HaBechira ch 3) that the place of the mizbeyach is "mechuvan b'yoseir" -- it has to be precisely placed in exactly the correct spot. Being even a little off disqualifies the whole effort. This type of exacting measurement is impossible even given the best surveying tools and the evidence of what once was. The place of the mizbeyach is not subject to calculation -- it can only be revealed to us by a navi.