GMT programs read real-world coordinates and convert them to positions on a plot. This is achieved by selecting one of several coordinate transformations or projections. We distinguish between three sets of such conversions:
The next chapter will be dedicated to GMT map projections in its entirety. Meanwhile, the present chapter will summarize the properties of the Cartesian and Polar coordinate transformations available in GMT, list which parameters define them, and demonstrate how they are used to create simple plot axes. We will mostly be using psbasemap (and occasionally psxy) to demostrate the various transformations. Our illustrations may differ from those you reproduce with the same commands because of different settings in our .gmtdefaults4 file.) Finally, note that while we will specify dimensions in inches (by appending i), you may want to use cm (c), meters (m), or points (p) as unit instead (see the gmtdefaults man page).