Umbilicus (reference point)

[1] The standard city plan included two major thoroughfares, decumanus maximus and cardo maximus, intersecting at the umbilicus[2] thus starting the centuriation (surveying).

Not only it symbolized the birth of a city; a mundus cerialis, an underground chamber dedicated to the underground gods, was dug next to it as a part of the breaking ground for the city.

[2] The Roman idea of an absolute and unchangeable center of the city is related to the beliefs that the city is a permanent dwelling of gods, with both the umbilicus and the pomerium predestined by the divine forces; even if the city was physically destroyed, it was not forsaken for as long as the deities remained.

[citation needed] Umbilicus soli (the "ground navel") was the name for the reference point of the Roman surveying tool, the groma, located at the moving end of a swinging arm, with a plumb bob suspended underneath.

[5][6][7][8] The purpose of offsetting the reference point from the Jacob's staff (vertical pole) was twofold: it enabled sighting of lines on the ground through a pair of strings (used to suspend an opposite pair of plumbs from the cross) and allowed placing the reference point over a sturdy object (like a boundary stone), where the staff cannot be inserted.