Been studying for the test and am getting a lot out of it. I just barely followed the topics of 'datums', coordinate systems, projections, etc. during the class discussion and the first read-thru of the text. However, now it's getting much clearer.
I was astonished to learn that the earth's gravitational pull is not uniform at all spots on the earth. I was raised on Newton's law that gravity uniformly draws things toward the center of the earth. This is apparently not absolutely true, due to variations in the density of the earth.
I recall now that back in the 1960's during one of NASA's pre-moon-landing visits to the moon that the space module went into "wild gyrations" and started spinning out of control as it orbited the moon at fairly close range. I remember the announcer's panicked voice and some scientist surmising that there may be some giant meteor buried beneath the surface of the moon and it may have cause a blip in the gravitational field of the moon, thus triggering these wild gyrations.
The textbook points out that the surface of the ocean is 500 feet higher at some points than other at some points, being caused by these variations in earth's gravitational pull. I always thought that "sea level" was always a perfect constant number (other than minimal variations due to waves and storms).
Interesting, also, that the U.S. Land Survey was not based upon any coordinate system but was just based upon land measurements, in an attempt to establish some order as the west was settled. Also, that none of the original 13 colonies were part of it (they predated it.)