GEOG 862
GPS and GNSS for Geospatial Professionals

Datums (Reference Frames)

NASA image used Landsat data to texture-map surface created using SRTM Elevation data.
Part of the Earth's Lithosphere
Source: Wikipedia

The second part of the answer to the question posed earlier is this: if geographic coordinates are to have meaning they must have a context, a Reference Frame (Datum).

Despite the certainty of the physical surface of the Earth, the lithosphere, it remains notoriously difficult to define in mathematical terms. The dilemma is illustrated by the ancient struggle to represent its curved surface on flat maps. There have been a whole variety of map projections developed over the centuries that rely on mathematical relationships between positions on the Earth’s surface and points on the map. Each projection serves a particular application well, but none of them can represent the Earth without distortion. For example, no modern surveyor would presume to promise a client a high-precision control network with data scaled from a map. As the technology of measurement has improved, the pressure for greater exactness in the definition of the Earth’s shape has increased. Even with electronic tools that widen the scope and increase the precision of the data, perfection is nowhere in sight.