Decumanus is a term from the Roman land surveying . It describes the vertical lines in a rectangular surveying system.
Originally the term was used in cosmology and referred to the east-west axis, which was used as a line of sight when observing the movement of the heavens. The counterpoint is the north-south axis called cardo . The decumanus divided the sky into a day and a night, while the cardo divided the sky into a sunrise and a sunset.
With the surveying device called groma , a coordinate system was created that served as the basis for the other streets and parcels of the city to be created. In gromatic practice, a decumanus maximus (DM) was created, a main axis of orientation that was not based on astronomical, but topographical aspects. Usually the decumanus was laid out where the greatest possible expansion of a city was possible, or where there was already an existing street. The axis orthogonal to this was the cardo maximus (CM). These two main streets were used as a basis for new cities. The decumanus maximus was laid out at a width of 40 Roman feet (twelve meters). That always corresponded to twice the width of the cardo maximus .
The decumanus was also created during the construction of military camps and referred to as prorsi ("facing the enemy").
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