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Dividing the land into rectangular plots was used by the [[Ancient Greeks]], [[Ancient Egyptians|Egyptians]] and even [[Mesopotamians]].{{sfn | Lewis | 2001 | p=120}} However, the sheer scale of Roman [[centuriation]] from the 2nd century [[Anno Domini|B.C.]], when the new [[Roman colony|colonies]] were formed mostly to provide for veterans and landless citizens, was unprecedented,{{sfn | Lewis | 2001 | p=121}} so it is not clear to what extent Greek practices influenced the Roman surveyors.{{sfn | Lewis | 2001 | p=121}} The peculiarities of the Roman surveying methods and terminology suggest independence of Roman measurement tradition.{{sfn | Lewis | 2001 | p=123}}
Dividing the land into rectangular plots was used by the [[Ancient Greeks]], [[Ancient Egyptians|Egyptians]] and even [[Mesopotamians]].{{sfn | Lewis | 2001 | p=120}} However, the sheer scale of Roman [[centuriation]] from the 2nd century [[Anno Domini|B.C.]], when the new [[Roman colony|colonies]] were formed mostly to provide for veterans and landless citizens, was unprecedented,{{sfn | Lewis | 2001 | p=121}} so it is not clear to what extent Greek practices influenced the Roman surveyors.{{sfn | Lewis | 2001 | p=121}} The peculiarities of the Roman surveying methods and terminology suggest independence of Roman measurement tradition.{{sfn | Lewis | 2001 | p=123}}


The groma surveying instrument may have originated in [[Mesopotamia]] or [[Greece]] before the 4th century BC. Subsequently, it was brought to Rome by the [[Etruscan civilization|Etruscans]] and named '''cranema'''.<ref name="YanCeccarelli2009">{{cite book|last1=Yan|first1=Hong-Sen|last2=Ceccarelli|first2=Marco|title=International Symposium on History of Machines and Mechanisms: Proceedings of HMM 2008|url=https://books.google.com/books?id=Dkq6_mdW43IC&pg=PR5|accessdate=28 June 2014|date=2009-01-11|publisher=[[Springer Science+Business Media|Springer]]|isbn=9781402094859|page=107}}</ref>
The groma surveying instrument may have originated in [[Mesopotamia]] or [[Greece]] before the 4th century BC. Subsequently, it was brought to Rome by the [[Etruscan civilization|Etruscans]] and named '''cranema'''.{{sfn|Russo|Rossi|Ceccarelli|Russo|2009|loc=Topographic Instruments}}


== Limitations ==
== Limitations ==

Revision as of 07:29, 30 September 2023

Groma

The groma (as standardized in the imperial Latin, sometimes croma, or gruma in the literature of the republican times)[1] was a Roman surveying instrument.[2] The groma allowed projecting right angles and straight lines and thus enabling the centuriation (setting up of a rectangular grid). It is the only Roman surveying tool with examples that made it through to the present day.[3]

Construction

Groma. D points to the umbilicus soli

The tool utilizes a rotating horizontal cross with plumb bobs hanging down from all four ends.[4] The center of the cross represents the umbilicus soli[5] (reference point).[6] The cross is mounted on a vertical Jacob's staff,[4] so called ferramentum.[6] The umbilicus is offset with respect to the ferramentum by using a bracket pivoting on the top of the staff (frequently ferramentum is used to describe the whole tool).[6] The purpose of offsetting the reference point from the Jacob's staff (vertical pole) is twofold: it enables sighting of lines on the ground through a pair of strings (used to suspend an opposite pair of plumbs from the cross) without the staff obscuring the view and allows placing the reference point over a sturdy object (like a boundary stone), where the staff cannot be inserted.[7]

Bracket controversy

The pivoting bracket on the top of the staff was suggested in the 1912 reconstruction by Adolf Schulten and confirmed by Matteo Della Corte [it] soon afterwards.[8] However, as asserted by T. Schiöler in 1994,[9] the 5-kilogram cross found in Pompeii is too heavy and thus the bracket never existed. Furthermore, there is no archeological evidence of the bracket, and the images of gromas on tombstones do not show it.[10] The archeologists rejecting the backet suggest that the staff was slightly angled to permit sighting without the pole obscuring the view.[11]

Use

Despite a lot of surviving information about the groma (and the extreme simplicity of toll itself[12]), the details of its operation are not clear.[4] The general idea is straightforward:[4] the staff was inserted into the ground a bracket length away from the marker. The bracket was swung so that the umbilicus soli was directly on top of the center of the marker.[6] The cross was then turned to align with the desired directions and surveyor's assistant would step back and place a pole as directed by the surveyor (a gromaticus[13]) who was sighting the pole through two strings on the opposite ends of the cross.[14]

The distances were measured using rods. The setup works on the level ground or gentle slopes; the details of a survey crossing a steep-sided valley are not clear.[1]

History

The name "groma" came to Latin from the Greek gnoma via the Etruscan language. It is unclear, which of the many meanings of the Greek gnomon ("gnoma" is a form) was used, although multiple sources to designate the central point of a camp or town.[1]

Dividing the land into rectangular plots was used by the Ancient Greeks, Egyptians and even Mesopotamians.[12] However, the sheer scale of Roman centuriation from the 2nd century B.C., when the new colonies were formed mostly to provide for veterans and landless citizens, was unprecedented,[15] so it is not clear to what extent Greek practices influenced the Roman surveyors.[15] The peculiarities of the Roman surveying methods and terminology suggest independence of Roman measurement tradition.[16]

The groma surveying instrument may have originated in Mesopotamia or Greece before the 4th century BC. Subsequently, it was brought to Rome by the Etruscans and named cranema.[17]

Limitations

As stated in the contemporary literature (and confirmed in the modern experiments), the alignment of the plumb-lines of the groma is quite susceptible to wind. This compares unfavorably with dioptra.[18] Also, the far plumb-line on the cross is optically thinner than the closer one, introducing the angle error calculated by the archeologists to be about 1.5 promille (about 1 meter per the side of centuria, 710 meters).[19]

Notes

  1. ^ a b c Lewis 2001, p. 125.
  2. ^ Cesare Rossi, Marco Ceccarelli, Michela Cigola, The groma, the surveyor's cross and the chorobates. In-depth notes on the design of the old instruments and their use. La Groma, lo Squadro agrimensorio e il corobate. Note di approfondimento su progettazione e funzionalità di antiche strumentazioni, in Disegnare Idee Immagini, anno XXII n. 42/2011; pp. 22-33. ISBN 978-88-492-2248-7, ISSN IT 1123-924
  3. ^ Lewis 2001, pp. 124–125.
  4. ^ a b c d Lewis 2001, p. 124.
  5. ^ Per Lewis, this is a "strange name".
  6. ^ a b c d Lewis 2001, p. 126.
  7. ^ Stone 1928, pp. 225–226.
  8. ^ Lewis 2001, pp. 127–128.
  9. ^ Schiöler, T. (1994) ‘The Pompeii Groma in New Light’, Analecta Romana, 22, pp. 45-60.
  10. ^ Lewis 2001, pp. 128–129.
  11. ^ Lewis 2001, p. 131.
  12. ^ a b Lewis 2001, p. 120.
  13. ^ Lewis 2001, p. 122.
  14. ^ Lewis 2001, pp. 126–127.
  15. ^ a b Lewis 2001, p. 121.
  16. ^ Lewis 2001, p. 123.
  17. ^ Russo et al. 2009, Topographic Instruments.
  18. ^ Lewis 2001, p. 132.
  19. ^ Lewis 2001, p. 133.

Sources

  • Lewis, M. J. T. (2001-04-23). "The groma". Surveying Instruments of Greece and Rome. Cambridge University Press. pp. 120–133. doi:10.1017/cbo9780511483035.008.
  • Stone, Edward Noble (1928). "Roman Surveying Instruments". University of Washington Publications: Language and literature. University of Washington Publications: Language and Literature. 4 (4). University of Washington Press: 215–242. Retrieved 2023-09-27.
  • Russo, Flavio; Rossi, Cesare; Ceccarelli, Marco; Russo, Ferruccio (2009). "Devices for Distance and Time Measurement at the Time of the Roman Empire". International Symposium on History of Machines and Mechanisms: Proceedings of HMM 2008. History of Mechanism and Machine Science. Springer Netherlands. p. 107. ISBN 978-1-4020-9485-9. Retrieved 2023-09-30.

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