I've heard the Roman roads are amazingly straight. They achieved this through an early surveying system where they used a tool called a groma. It's pretty simple to make really. In fact we had the supplies to make a rough one in our garage. To better understand the method the Romans used to build their roads and survey their land, my daughter and her friend built a groma and used it to lay out a right angle in our yard.
There are a couple of excellent youtube videos of how to make a groma as well as how to use one. The one we had the girls watch before attempting to build or use their own groma is here. This just explains the use of one. My hubby led them in the assembling of the groma.