How to build a road perfectly strait for an indefinite number of miles(only in East West direction).

I watched a show the other day about Mayan civilization on the Discovery Channel. They said scientists could not figure out how they built perfectly straight roads for 60 miles that would not very by even a degree, with the technology of the time. Although this is not a perfect solution and it does not account for varying elevation of the surface of the land. I thought it might be a unique solution to this problem with more general applications.

Here is my idea.

Set a tall pole exactly perpendicular to the earth’s surface using a plum line. Mark the end of the shadow throughout the day with a string keeping the line on the earth as you go. The next day Move the pole to a new point further along the path and reset the pole with the plum line making sure that the shadow matches up with the line already placed on the earth. Continue until the road has been measured to desired length.