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The Groma or gruma (altered from Greek gnomon γνόμων "indicator", possibly through Etruscan) was the principal Roman surveying instrument. It comprised a vertical staff with horizontal cross pieces mounted at right-angles on a bracket. Each cross piece had a plumb line hanging vertically at each end. It was used to survey straight lines and right-angles, thence squares or rectangles. The same name was given to:

  • the center of any new military camp, ie the point from which was traced the regular grid by using the groma instrument
  • the center of a new town, from which the gromatici (surveyors) began to lay out cardo and decumanus grid, with a plough and a pair of oxen

The groma surveying instrument originated from Mesopotamia, from where it was imported by the Greeks in the 4th century BC. Subsequently, it was brought to Rome by the Etruscans.[1]


  1. ^ Hong-Sen Yan & Marco Ceccarelli (2009), International Symposium on History of Machines and Mechanisms: Proceedings of HMM 2008, Springer, p. 107, ISBN 1402094841 


  • University of Pennsylvania - Corinth Computer Project

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