I hadn’t seen that cross with plumb bobs on it before. Searched and found this for anyone else interested.
For road building and laying out settlements in the distinctive grid patters, the main tool of agrimensores was a device known as the groma, believed to have had its origins in Egypt. This instrument consisted of a long wooden staff, pointed at the lower end and topped with a wooden arm, about 25cm/10" long. This cross arm had a pivot to support the stellata, which was the main part of the instrument and consisted of a wooden cross with plumb bobs at the end. To use the groma, the surveyor set it up where two roads were to intersect at right angles, and used the plumb bobs as guides for laying out a line of stakes or flags. Using the naked eye, these stakes could be constantly realigned, to maintain a straight line as the course of the road was plotted. Source
