The Groma was an ancient Roman instrument for quantity surveilling and measuring out field boundaries.
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Ancient Roman topography was highly effective with highly skilled technicians using relatively simple instruments. The Groma was an ancient Roman instrument to measure out field boundaries. It was more basic in operation than the Dioptre, which allowed precise angle measurements. However it was a small piece of technology with profound significance for Roman development.
The religious importance of field and city boundaries
In the earliest days of Rome, field or city boundaries had a very significant and religious role. The myth of Romulus and Remus tells us that one brother killed the other for mocking the boundaries of the new city. The god Janus, from whom the Roman calendar month January is dedicated oversaw all boundaries. It is not surprising that the early Groma was used by the Augurs. As well as overseeing sacrifices and fortune telling they identified the propitious sites and orientation of early Roman settlements.
Until the early part of the 20th century its only image and possible reconstruction was through a funerary tomb stone of the land surveyor Lucius Aebutius Faustus. A complete one was then found at Pompeii which allowed its full reconstruction.
Description of a simple Groma
The sketch below outlines what an ancient Roman Groma looked like. It was regularly used by surveyors for mapping out ownership boundaries, or subdivision of public lands for retiring soldiers.
The vertical post into the ground was called “ferramentum”. The orthogonal staffs were called “cornicula”. Plumb lines called “nerviae”, were hung with weights on their ends called “pondera”. We can also see a central plumb line which hung off a pivoted link from the ferramentum to the cross-staffs. This was called “umbilicus solis”. The central plumb line allowed the surveyor to position the centre of the instrument above the measuring point.
How the Groma was used to plan out a new city.
Laying out the road network as basic structure
The layout of a new colony or settlement started with the Groma aligned N-S and W-E. This allowed the main trunk roads to be laid out : the Cardo Maximus and Decumanus Maximus. Extension of these straight lines of sight was achieved by the use of aligned posts. These major roads were thus laid out orthogonal to one another and would lead all the way, dead straight to the future main gates of the new city. The city’s forum would often be laid out at the crossing of these main roads, essentially the centre of the city.
At this point the two main trunk roads could be subdivided again and again to form a grid:
- secondary roads called “limites quintarii” at a set distance of 100 “actus”. These larger grids were termed “saltus”.
- tertiary roads were then dropped in at smaller regular intervals of about 20 actus. These smaller grids were termed “centuries“
It is worth noting that a that a Roman foot was about 0.3m and that an actus = 120 Roman feet ie ~35-40m. Hence limites quinarii were at about 3.5km and centuries were about 750-800m in width.
The three types of roman road had set widths in Roman feet (called “pes”): 40ft (~12m), 20ft (~6m), 12ft (3-4m) and lastly 8ft (~2m) for other smaller streets and alleys.
Subdividing the land areas within the road structure
Having laid out the overall street network the surveyor could move on to subdividing the land areas between them. The centuries were subdivided into 10×10 squares 2acta (~75m) width each.
Measuring the distance to inaccessible points
Experts could use the Groma to great effect to enable topographical mapping. Its ability to accurately lay out orthogonal directions allowed surveyors to even measure the distance to inaccessible points: This was achieved by creating identical triangles, reflecting the distant point to an accessible location which could be directly measured.
Read more about it in our page about Ancient Roman Maps