Plan D. Busson and V. Charlanne. Cliché
IGN Photothèque Nationale - 1999.
||The basic unit of measurement
The discovery of ancient roads, the reconstruction of the boundaries of the main Early Roman monuments and the path of certain medieval roads have all contributed to the establishment of a theoretical, strictly orthogonal town grid. The basic unit of this grid has been found everywhere in the central section of Lutetia.
This unit corresponds to exactly 300 Roman feet, with occasional subdivisions of 150 feet. The choice of this unit proves that the urban plan of Lutetia was an abstract procedure, and that the planners did not have to consider previous settlements-or chose not to. This grid system seems also to have been partially used on the right bank, even in areas that were not completely urbanised. Thus, between the cardo maximus (rue Saint-Martin) and the cardo that is the present Rue Saint-Denis it is exactly 600 Roman feet.
Laying out the grid
To implement this grid system, the surveyors probably established a straight line that would become the principal axis of the settlement, the cardo maximus. We can imagine the initial location of the groma would have been the highest point on this axis, one that offered a very clear view of the island and the right bank. This "zero point" was probably located at the southwest corner of the forum, which corresponds to nos. 172 and 174 of Rue Saint-Jacques: the highest point on the Saint-Geneviève hill.