# Groma

Measuring chain and angle cross: After Carolus Stephanus and Johannes Liebhaltus, Strasbourg 1579.
Replica of a groma
Drawing reconstruction

The Groma ( Greek γρῶμα ) was a Roman surveying instrument (combination of plumb line and crosshair ), forerunner of the double pentagon prism , designed for staking out right angles . Latin names have also been passed down for the Groma: ferramentum , iron device, it was called, like many devices, after the base material of its base, tetrans 'quadrant' after the exact name for the intersection of its beams, and stella because of the similarity of the cross shape of its sight ruler a 'star'.

## Appearance and functionality

The appearance and functioning of the groma can be easily reconstructed based on a find in Pompeii as well as from the representation on ancient grave steles from Ivrea and Pompeii.

A rotatable, if possible right-angled, axis cross is attached to a cranked rod stand , at the ends of which a total of four perpendiculars hang down. The extension arm allows the intersection of the axis cross with another perpendicular to be centered over a measurement point. Models without an extension arm, but with a viewing slit in the tripod rod, were also found on illustrations. On the diagonally opposite plumb one is , draw attention possible. For precise work it is not necessary that the groma is set up vertically and thus the cross is parallel to the ground, since only the plumb bobs, which always hang down vertically, are used for the measurement. For the measurement, the groma was set up with the center plumb line over a measuring point and set up on a baseline by means of a sight over the first pair of plumb lines. Since the axis cross should be right-angled, the sight over the other pair of plumb lines shows a line perpendicular to it. The arrangement of the plumb bobs on the axis cross allows errors to be compensated for by means of a second control stakeout: if the axis cross is rotated 90 ° and the setup and measurement repeated, the second measurement will show a deviation due to the (inevitable) device error. The staking error can be halved on average.

So that the control measurement was not accidentally made using the plumb bobs of the first measurement, the plumb bobs were given a different shape in pairs.

## application

The groma was used in particular for the Limitation , the creation of a legionary camp or of settlements to determine the course of the main roads. The location of the first survey was the locus gromae or the umbilicus 'navel' and goes back to the “quartering of the observation area” by the augurs .

"The Augur divides his field of vision into four regions, left and right, front and back, by aiming at prominent points on the horizon as boundary marks and drawing lines in the air on the corresponding sides with his crook ."

The availability of a right angle in the terrain also enabled more extensive applications, such as the precise planning of tunnels or even the measurement of rivers without crossing them, as described by the ancient author Marcus Iunius Nipsus .

## Finds

All iron found objects that were interpreted as groma in the past and some of which were found in connection with Roman warehouses ( Horrea ) have now been identified by research as the remains of a measuring vessel ( Modius ). The basis for this was the deliberations published in 1994 by the archaeologist Dietwulf Baatz on an alleged groma from Fort Pfünz . The iron ax cross in the holdings of the Roman Department of the Museum of Prehistory and Early History on the Willibaldsburg is addressed as part of a groma.

The most important evidence for the research and reconstruction of this archaeologically extremely seldom proven Roman device for the measurement of buildings and properties are sparse finds and images. Only the following mentions are worthy of discussion:

• Ivrea : In 1852 the tombstone of a mensor (surveyor) from the 1st century BC Found. In addition to the materials of the mensor, the shaft and cross of a groma are also carved; the stone is kept in Ivrea.
• Pompeii : A metal groma was found in Pompeii in 1912. The good state of preservation, the solid bronze work and the completeness make up the significant value of the find. The arms of the Pompeiian Stella measure 46 cm from the pivot point. The device was reconstructed in the 1920s by Matteo della Corte who, like Baatz, clearly identified the rotating cross found in the granary of Fort Pfünz as part of a grain measure.
• Fayyum : A wooden cross known since 1899 is said to come from Ptolemaic times, i.e. 100 AD, which was found in Fayyum, is now kept in London and was addressed in the past as part of a groma. However, this interpretation is very controversial, because a mode is also assumed in this find .
• Coins: The sighting instrument may be depicted on a copper coin from the Lucanian Herakleia . The depiction on a silver coin from Metapont , once interpreted as a groma, has long been interpreted as a cross torch .

## literature

• Klaus Grewe : Chorobat and Groma. New thoughts on reconstructing and handling the two most important surveying devices used by ancient engineers. In: Bonner Jahrbücher . Vol. 209, 2009, pp. 109-128.
• Nikolaus Thurn : The birth of the theory from the instrument. About the use and meaning of the ancient instruments groma and lyre. Fink, Paderborn et al. 2008, ISBN 978-3-7705-4474-5 .
• 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 Idea Immagini, anno XXII n. 42/2011; pp. 22-33. ISBN 978-88-492-2248-7 , ISSN IT 1123-924

## Individual evidence

1. Paul of Naredi-Rainer: Architecture and harmony. Number, measure and proportion in western architecture. 5th, revised edition. DuMont, Cologne 1995, ISBN 3-7701-3523-7 , p. 69, note 153.
2. ^ Margot Klee: The Roman Limes in Hesse. History and sites of the UNESCO World Heritage. Friedrich Pustet, Regensburg 2009, ISBN 978-3-7917-2232-0 , pp. 41-42.
3. Dietwulf Baatz : Groma or Modius? A find from the Limes Fort Pfünz. In: Bavarian history sheets . Vol. 59, 1994, pp. 73-83.
4. ^ Museum of Prehistory and Early History on the Eichstätt Historical Association
5. ^ Matthias Pausch: End of a misinterpretation. The Roman groma from Pfünz. In: Ancient World . Vol. 29, No. 6, 1998, pp. 541-544.
6. ^ Matthias Pausch: End of a misinterpretation. The Roman groma from Pfünz. In: Ancient World. Vol. 29, No. 6, 1998, pp. 541-544, here p. 542.
7. ^ Konrad Schauenburg : Helios. Archaeological and mythological studies of the ancient sun god. Published by the German Archaeological Institute. Gebr. Mann, Berlin 1955, p. 75.
8. Friedrich von Schrötter (Ed.): Dictionary of coinage. 2nd, unchanged edition. Walter de Gruyter, Berlin 1970, p. 764.