Ancient Roman transportation engineers built a vast network of roads that stretched across Europe, North Africa and parts of the Middle East. This complex road system was a key to travel and trade in the Roman Empire, and many of the centuries-old streets had a reputation for being extremely straight, although not all of them were.

For instance, Via Appia (Appian Way), which connected Rome to the port of Brundisium in southern Italy, was more than 300 miles (500 kilometers) long, and sizable parts of it were straight. Another Roman road, Stane Street in southern England, was built to connect London to Chichester. Much of the road, which stretches roughly 57 miles (92 km), is straight. The Middle East also had straight Roman roads, including a coastal avenue from Antioch, Turkey, to what is now Gaza. A recent mapping project was able to map about 186,400 miles (300,000 km) of roads and more are probably undiscovered. But how did the Romans avoid unnecessary twists and turns to ensure straight streets? The answer may lie in three surveying instruments the Romans used.

In some instances, the Romans built on top of older roads that existed before they conquered an area. Their “road network incorporated older roads from a broad range of different societies and polities,” Marion Kruse, an associate professor of classics at the University of Cincinnati, told Live Science in an email. But when the Romans built new roads, they used a few different tools to help plan them.

“Three instruments were used consistently by Roman road builders: the dioptra, the groma, and the chorobatus [or chorobates],” Adriana Panaite, a researcher at the Vasile Pârvan Institute of Archaeology in Romania who has studied Roman roads extensively, told Live Science in an email.

While the dioptra is known from ancient texts, no example of it has ever been found in an archaeological dig, according to M.J.T. Lewis, who was a historian at the University of Hull in the U.K. In his book “Surveying Instruments of Greece and Rome” (Cambridge University Press, 2001), Lewis noted that the design of the dioptra varied considerably. The different designs tended to include a stand and a disc-shaped base with a tube-shaped sighting instrument attached. An ancient surveyor could look through the tube and see a distant object without extraneous light interfering, allowing for a better view.