3 Spatial reference systems

 

This chapter covers

  • What spatial reference systems are
  • How to determine the spatial reference system of a dataset
  • How to choose appropriate spatial reference systems

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.

This chapter begins by discussing different types of spatial reference systems (SRSs). It then follows up with sections on choosing suitable SRSs and uncovering the SRS of the source data when it is not apparent.

3.1 Spatial reference systems: What are they?

3.1.1 Geoids

3.1.2 Ellipsoids

3.1.3 Datum

3.1.4 Coordinate reference system

3.1.5 Spatial reference system essentials

3.1.6 Projections

3.2 Selecting a spatial reference system for storing data

3.2.1 Pros and cons of using EPSG:4326

3.2.2 Geography data type for EPSG:4326

3.2.3 Mapping just for presentation

3.2.4 Covering the globe when distance is a concern

3.3 Determining the spatial reference system of source data

3.3.1 Guessing at a spatial reference system