The Groma or gruma was a 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, i.e. 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 may have originated from Mesopotamia, and may have been imported by the Greeks in the 4th century BC. Subsequently, it was brought to Rome by the Etruscans and named cranema or ferramentum.
- Lewis, M. J. T. (2001-04-23). Surveying Instruments of Greece and Rome. McGraw Hill Professional. p. 120. ISBN 9781139430357. Retrieved 29 June 2014.
- Yan, Hong-Sen; Ceccarelli, Marco (2009-01-11). International Symposium on History of Machines and Mechanisms: Proceedings of HMM 2008. Springer. p. 107. ISBN 9781402094859. Retrieved 28 June 2014.