As explained in the overview, for this sundial to work properly and maintain a one minute accuracy it had to be designed and built for this specific location at Mount Auburn Cemetery.
At the center of the sundial is a gnomonic projection map that is generated by projecting points on the Earth’s surface to a flat plane that touches the Earth at a specific location which then becomes the map's center point.
A map of this type has several interesting features. Unlike most commonly viewed maps, any straight line connecting two points on the map is the shortest distance between those points. Any straight line is also a part of a great circle which is defined as a circle on the Earth’s surface whose center is coincident with the Earth’s center. This is why the equator appears as a straight line as it is a great circle as are all the lines of longitude radiating from the small, dome shaped gnomon support.
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William Andrewes, who invented this type of sundial, noticed that on a map of this kind the lines of longitude radiating from the the North Pole have the same spacing as the hour lines on a conventional sundial. By overlaying one on top of the other you end up with his unique dial design that combines both time and space. The story behind the invention of the Longitude Dial and