Once your data knows where to draw, it needs to know how.
You still need to know which GCS it is in before you know where it is on Earth. The coordinates 134.577☎, 24.006°S only tell you where a location is within a geographic coordinate system. The GCS is what ties your coordinate values to real locations on the earth. Australian Geodetic Datum 1984 is designed to fit the earth snugly around Australia, giving you good precision for this continent but poor accuracy anywhere else. There are many different models of the earth’s surface, and therefore many different GCS! World Geodetic System 1984 (WGS 1984) is designed as a one-size-fits-all GCS, good for mapping global data. But in order to draw a graticule, you need a model of the earth that is at least a regular spheroid, if not a perfect sphere. Because the planet spins, the poles are a bit closer to the center of the earth than the equator is. There are high mountains and deep ocean trenches. It’s a lumpy, bumpy, and uneven rounded surface.
Well it turns out the earth isn’t a perfect sphere. So why isn’t knowing the latitude and longitude of a location good enough to know where it is? How can location A and location B in the Australia example both be correct?
