Three distinct figures are involved in a geodetic datum for latitude, longitude and height: the geoid, the reference ellipsoid, and the Earth itself. Due in large measure to the ascendancy of satellite geodesy, it has become highly desirable that they share a common center.
While the level surface of the geoid provides a solid foundation for the definitions of heights and the topographic surface of the Earth is necessarily where measurements are made, neither can serve as the reference surface for geodetic positions. From the continents to the floors of the oceans, the solid Earth's actual surface is too irregular to be represented by a simple mathematical statement.
The geoid, which is sometimes under, and sometimes above, the surface of the Earth, has an overall shape that also defies any concise geometrical definition. But the ellipsoid not only has the same general shape as the Earth, but, unlike the other two figures, can be described simply and completely in mathematical terms.
A good understanding of these ideas is essential to clear comprehension of satellite based positioning. The next lesson will focus on the most usual manifestations of GPS coordinates in the United States, State Plane Coordinates and UTM coordinates. We will also have a bit of discussion on an often misunderstood topic- heights.
Before you go on to Lesson 6, double-check the Lesson 5 Checklist to make sure you have completed all of the activities listed there.