A Roman groma
Four weighted strings hung from a cross on a staff. A Roman surveyor sighted along two of them to lay out a right angle, then did it again. Conquered land from Italy to North Africa was cut into squares this way, and in places the grid can still be seen from the air. The surveyor worked out the lines. Soldiers and enslaved workers did the digging.
Found: Pompeii, 1912: the metal parts of one groma, in a workshop attributed to a surveyor named Verus.
What it was
A staff with a horizontal cross at the top and a plumb-line hanging from each of its four arms. Line up two opposite strings with a marker pole in the distance and you have a straight line; turn to the other pair and you have a line at exactly ninety degrees to it. The Roman land-surveying manuals are full of it, and the surveyors were named after it: gromatici.
What it was for
Right angles, mostly: the corners of army camps, town blocks, and above all centuriation, carving newly taken land into squares about 710 metres a side to hand out to settlers, very often discharged soldiers. The surveying was a way of taking land over.
The one that survives
Only one groma has ever been found: metal fittings excavated at Pompeii in 1912, buried by the eruption of AD 79. The wooden staff and arms were gone. How the cross was mounted on the staff has been argued ever since. The usual reconstruction swings it out on a bracket, and at least one specialist has argued the cross is too heavy for that bracket ever to have held it.
- Hyginus Gromaticus, Constitutio limitum, in the Corpus Agrimensorum Romanorum
- Dilke, The Roman Land Surveyors (1971)
- Lewis, Surveying Instruments of Greece and Rome (2001)
- Picture: made with AI for The Ember Atlas, an imagining, not a record of how it looked.