The user may define subclasses of this class.
The horizontal and vertical components of the coordinate system are defined individually.
For analytical purposes, such as defining the coordinates of a new point on an arc by interpolation of adjacent, pre-existing points, an assumption must be made about the nature of the space on which or in which the arc resides. Typically the space is assumed to be planar for these purposes. Where large distances are involved, it may be justified to consider the space as curved.