Chorobates
This instrument was very long (about 3 m). In the middle was a groove. Like a spirit level it contained water. The instrument was used for measuring the angle of the trench/aqueduct.  

Dioptra
This is a surveying instrument. A straight line could be measured with it. It was not easy to use it for long stretches. The "librator" had to move the instrument hundreds of times between source and town. He calculated the angleof the aqueduct by using the supposed length of the aqueduct and the difference in height between source and town.

Leveling staff
Surveying instruments like the dioptra and levelling staff were essential in laying out a proposed path for a new aqueduct.