Groma surveying

The Groma was a Roman surveying instrument ( combination of solder and visor Cross), precursor of the double penta prism, designed to stake a right angle. The name comes from the Greek ( γρῶμα ), but there are also Latin names handed down: ferramentum ( " iron Device") it was said that as many device, according to the raw material of his pedestal, Tetrans ( "Quadrant " ) for the exact name for the intersection its beams, and stella because of the similarity of the cruciform his visor ruler with a " star".

Looks and works

Appearance and functionality of the Groma can be due to a discovery in Pompeii, and the presentation on ancient grave stelae from Ivrea and Pompeii well reconstructed.

On a cranked rod tripod a rotatable rectangular coordinate system is attached as possible, hanging at the ends of four solders. The extension arm allows the intersection of the coordinate system with an additional lot could be centered over a survey point. There were also models without side arm, with a slit in the stand rod found this on pictures. About the diagonally opposite a plumb to sight is possible.

For an accurate operation, it is not necessary that the Groma is placed vertically, and thus the cross is parallel to the ground, since only the alloys which always hang vertically, can be used for the measurement.

To measure the Groma was set up with the bisector on a survey point, and furnished by, draw attention on the first pair of plumb lines on a baseline. Since the coordinate system should be square, shows to sight over the other pair to plumb a perpendicular line.

The arrangement of solder on the cross axis error compensation allows a second Kontrollabsteckung: If the coordinate system that is rotated 90 ° and repeat the preparation and measurement, is shown in the second measurement by the ( inevitable ) device error deviation. The Absteckfehler can be halved on average.

At this point the measurement was not made ​​accidentally pass the solders of the first measurement, we had pairs given perpendiculars a different shape.

Application

The Groma was especially used in the limitation, the creation of a military camp or settlements, to set 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 " fourfold division of the observation space " by the augurs.

However, the availability of a right angle in the field has also enabled more advanced applications, such as the precise planning of tunnels or even the surveying of rivers, without crossing them, as it is described by the ancient author Marcus Junius Nipsus.

Finds

All iron found objects that have been interpreted in the past as Groma and partly in Fund contexts with Roman granaries ( Horrea ) were found, has identified the research now as remnants of a Hohlmaßgefäßes ( modius ). The basis for this were the considerations published in 1994 by the archaeologist Dietwulf Baatz to an alleged Groma from the fort Pfünz.

The most important indication for the study and reconstruction of this archaeologically proven extremely rare Roman device for the measurement of buildings and land are scarce finds and pictures. Discussion are worthy of only the following mentions:

• Ivrea: 1852, the grave stone of a Mensors ( surveyor ) is found from the 1st century BC. In addition to materials of the Mensors also the stem and the cross a Groma are carved; the stone is preserved in Ivrea.
• Pompeii: 1912 a metal Groma was found in Pompeii. The good state of preservation, the dignified bronze work and the completeness make the significant value of the find. The arms of the Pompeian Stella measure from the pivot point of each 46 cm. The device was reconstructed in the 1920s by Matteo della Corte, like Baatz clearly identified the located in the granary of the fort Pfünz rotating the cross as part of a grain measure.
• Fayyum: From the Ptolemaic period, ie 100 AD to a well-known since 1899 wood cross date, which was found in the Fayyum, is now kept in London and has been addressed in the past as part of a Groma. This interpretation, however, is very controversial, because even in this find a modius is conjectured.
• Coins: Maybe the visor tool is shown on a copper coin of luca African Heraclea. The once interpreted as Groma presentation on a silver coin of Metapontum has long been interpreted as a cross flare.