A Great Zero is a point within every new Age. It was set by the Ronay and D'ni Surveyors who first explored the Age, and established it usually on a prominent natural landmark. From the Great Zero a line was drawn, usually toward magnetic north or another prominent landmark, to aid in exploration, navigation, construction, etc.
Ri'neref chosen the line of the Great Zero of D'ni to point toward the magnetic North of the Earth. King Me'erta suggested it would rather point toward another prominent landmark. This was simply to question the beliefs of Ri'neref and to encourage the cults to build in a newer section of the D'ni city; he did not officially change the line, something that would have grave implications upon the culture. Upon the new line several new temples were built, including the Temple of the Tree, the Temple of Water and the Temple of the Sacred Stone.