The Great Zero itself was usually based on a prominent natural
landmark within an Age. From the Great Zero a line was drawn,
usually toward magnetic North, to aid in navigation, construction,
etc…(Note that the D’ni measurement and spatial system
was based on a polar coordinates, rather than rectangular.)
As had always been the case with the Ronay, a group of surveyors
was sent to D’ni, before the group officially moved there,
to establish the Great Zero and the line emanating from it. A
monument was built on the Great Zero in the year 0 DE.
Unlike, previous occasions, Ri’neref established the line
of the Great Zero as set apart for holy buildings. Without authorization
by the reigning King, construction was forbidden.
Though it’s never stated directly, records strongly imply
that it was Ri’neref who chose where the city would be established.
He seemed to base his decision on two factors (which probably
made the decision an easy one). First was the line of the Great
Zero. It seemed an obvious spot to base the city, with the most
important religious structures being directly on the line and
the rest of the city surrounding its center. The second factor
was a group of waterfalls that flowed from the ceiling of the
cavern to an area adjacent to the line of the Great Zero. The
fresh flowing water was perfect for drinking.