Up to this point, we’ve been working mostly with fictitious data. Using sample data to learn the basics of PostGIS is an excellent first step: you’re immediately rewarded with results without facing the messiness of real-world data. But reality is messy. From this chapter forward, you’ll confront the fact that the earth is not the nice beachball that you thought it was. The same exact point on the face of the earth could be described differently by different groups. With time, the point shifts and the accuracy of our instrumentation gets better.
One big conundrum facing geographers is that an absolute frame of reference is missing. Your frame of reference is the earth itself, on which you’re trying to establish absolute positions. The earth spins, tilts, and sometimes even wobbles. The measurements you take of the earth are not durable. The best you can do is to make known how and when you made your measurements.