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Groma (surveying)

January 01, 1970

For other uses, see Groma.

The groma (as standardized in the imperial Latin, sometimes croma, or gruma in the literature of the republican times)[1] was a surveying instrument used in the Roman Empire.[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]

A groma

History edit

The name "groma" came to Latin from the Greek gnoma via the Etruscan language. It is unclear which of the many meanings of the Ancient Greek: γνώμων gnomon (cf. Liddell & Scott,[4] "gnoma" is a form) was used, although in multiple sources the Greek term is used 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.[5] However, the sheer scale of Roman centuriation from the 2nd century BC, when the new colonies were formed mostly to provide for veterans and landless citizens, was unprecedented,[6] so it is not clear to what extent Greek practices influenced the Roman surveyors.[6] The peculiarities of the Roman surveying methods and terminology suggest independence of Roman measurement tradition.[7]

The groma may have originated in Mesopotamia or Greece before the 4th century BC. Subsequently, it was brought to Rome by the Etruscans and named cranema.[8] There were apparently no improvements to groma introduced in Roman times: all writers on the subject clearly assumed the perfect familiarity of a reader with the tool.[9]

Construction edit

 
Groma. D points to the umbilicus soli

The tool utilizes a rotating horizontal cross with plumb bobs hanging down from all four ends.[10] The center of the cross represents the umbilicus soli (reference point).[11] The cross is mounted on a vertical Jacob's staff,[10] or ferramentum.[11] 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).[11] 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.[12]

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.[13] However, as asserted by Thorkild Schiöler in 1994,[14] the 5-kilogram cross found in Pompeii is too heavy to be supported in this way, thus the bracket had never existed. Furthermore, there is no archeological evidence of the bracket, and the images of gromas on tombstones do not show it.[15] The archeologists rejecting the bracket suggest that the staff was slightly angled to permit sighting without the pole obscuring the view.[16]

Use edit

Despite a great deal of surviving information about the groma (and the simplicity of the tool itself[5]), the details of its operation are not entirely clear.[10] The general idea is straightforward:[10] the staff was inserted into the ground a bracket length away from the marker, and the bracket was then swung so that the umbilicus soli was directly on top of the center of the marker.[11] The cross was then turned to align with the desired directions and the surveyor's assistant would step back and place a pole as directed by the surveyor (a gromaticus[17]). The surveyor could then view the pole through two strings on the opposite ends of the cross.[18]

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]

The alignment of the plumb-lines of the groma is quite susceptible to wind. This compares unfavorably with dioptra.[19] 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 (linear error of about 1 meter per the side of centuria, 710 meters).[20]

Notes edit

  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. ^ Liddell & Scott 1889.
  5. ^ a b Lewis 2001, p. 120.
  6. ^ a b Lewis 2001, p. 121.
  7. ^ Lewis 2001, p. 123.
  8. ^ Russo et al. 2009, Topographic Instruments.
  9. ^ Kelsey 1926, p. 259.
  10. ^ a b c d Lewis 2001, p. 124.
  11. ^ a b c d Lewis 2001, p. 126.
  12. ^ Stone 1928, pp. 225–226.
  13. ^ Lewis 2001, pp. 127–128.
  14. ^ Schiöler, T. (1994) ‘The Pompeii Groma in New Light’, Analecta Romana, 22, pp. 45-60.
  15. ^ Lewis 2001, pp. 128–129.
  16. ^ Lewis 2001, p. 131.
  17. ^ Lewis 2001, p. 122.
  18. ^ Lewis 2001, pp. 126–127.
  19. ^ Lewis 2001, p. 132.
  20. ^ Lewis 2001, p. 133.

Sources edit

  • 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.
  • Kelsey, Francis W. (July 1926). "Groma by Matteo della Corte". Classical Philology. 21 (3). The University of Chicago Press: 259–262. JSTOR 263160.
  • Liddell, Henry George; Scott, Robert (1889). "γνώμων". An Intermediate Greek-English Lexicon. Oxford: Clarendon Press.

External links edit

  •   Media related to Groma at Wikimedia Commons

January 01, 1970
groma, surveying, other, uses, groma, groma, standardized, imperial, latin, sometimes, croma, gruma, literature, republican, times, surveying, instrument, used, roman, empire, groma, allowed, projecting, right, angles, straight, lines, thus, enabling, centuria. For other uses see Groma The groma as standardized in the imperial Latin sometimes croma or gruma in the literature of the republican times 1 was a surveying instrument used in the Roman Empire 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 A groma Contents 1 History 2 Construction 3 Use 4 Notes 5 Sources 6 External linksHistory editThe name groma came to Latin from the Greek gnoma via the Etruscan language It is unclear which of the many meanings of the Ancient Greek gnwmwn gnomon cf Liddell amp Scott 4 gnoma is a form was used although in multiple sources the Greek term is used 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 5 However the sheer scale of Roman centuriation from the 2nd century BC when the new colonies were formed mostly to provide for veterans and landless citizens was unprecedented 6 so it is not clear to what extent Greek practices influenced the Roman surveyors 6 The peculiarities of the Roman surveying methods and terminology suggest independence of Roman measurement tradition 7 The groma may have originated in Mesopotamia or Greece before the 4th century BC Subsequently it was brought to Rome by the Etruscans and named cranema 8 There were apparently no improvements to groma introduced in Roman times all writers on the subject clearly assumed the perfect familiarity of a reader with the tool 9 Construction edit nbsp Groma D points to the umbilicus soli The tool utilizes a rotating horizontal cross with plumb bobs hanging down from all four ends 10 The center of the cross represents the umbilicus soli reference point 11 The cross is mounted on a vertical Jacob s staff 10 or ferramentum 11 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 11 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 12 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 13 However as asserted by Thorkild Schioler in 1994 14 the 5 kilogram cross found in Pompeii is too heavy to be supported in this way thus the bracket had never existed Furthermore there is no archeological evidence of the bracket and the images of gromas on tombstones do not show it 15 The archeologists rejecting the bracket suggest that the staff was slightly angled to permit sighting without the pole obscuring the view 16 Use editDespite a great deal of surviving information about the groma and the simplicity of the tool itself 5 the details of its operation are not entirely clear 10 The general idea is straightforward 10 the staff was inserted into the ground a bracket length away from the marker and the bracket was then swung so that the umbilicus soli was directly on top of the center of the marker 11 The cross was then turned to align with the desired directions and the surveyor s assistant would step back and place a pole as directed by the surveyor a gromaticus 17 The surveyor could then view the pole through two strings on the opposite ends of the cross 18 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 The alignment of the plumb lines of the groma is quite susceptible to wind This compares unfavorably with dioptra 19 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 linear error of about 1 meter per the side of centuria 710 meters 20 Notes edit a b c Lewis 2001 p 125 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 funzionalita 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 Lewis 2001 pp 124 125 Liddell amp Scott 1889 a b Lewis 2001 p 120 a b Lewis 2001 p 121 Lewis 2001 p 123 Russo et al 2009 Topographic Instruments Kelsey 1926 p 259 a b c d Lewis 2001 p 124 a b c d Lewis 2001 p 126 Stone 1928 pp 225 226 Lewis 2001 pp 127 128 Schioler T 1994 The Pompeii Groma in New Light Analecta Romana 22 pp 45 60 Lewis 2001 pp 128 129 Lewis 2001 p 131 Lewis 2001 p 122 Lewis 2001 pp 126 127 Lewis 2001 p 132 Lewis 2001 p 133 Sources editLewis 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 Kelsey Francis W July 1926 Groma by Matteo della Corte Classical Philology 21 3 The University of Chicago Press 259 262 JSTOR 263160 Liddell Henry George Scott Robert 1889 gnwmwn An Intermediate Greek English Lexicon Oxford Clarendon Press External links edit nbsp Media related to Groma at Wikimedia Commons Retrieved from https en wikipedia org w index php title Groma surveying amp oldid 1197767465, wikipedia, wiki, book, books, library,

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