We begin by defining our global Cartesian coordinate system as shown in
Figure 1.
All surfaces and particle trajectories are defined in this coordinate system.
Here, we shall use capital letters to denote quantities in the global Cartesian
system and bold notation to denote vectors.
Thus: 
, defining node N as shown.
Figure 1: Global Cartesian Triad.
Further, we shall be dealing with two-dimensional prismatic geometries which
are infinite in Z.
Thus, we define surfaces as shown in
Figure 2,
comprised of straight lines connecting nodes 
and 
(linear surfaces
are used to simplify particle tracing calculations---curved surfaces must be
approximated by a number of linear segments).
Note that, the surfaces, although mathematically described in two dimensions,
are actually three-dimensional.
Figure 2: Details of Surface L.
