A. Introduction

1. A Question of Dimensions

A formal reference system is one which covers an area large enough that the Earth's shape must be taken into account. It can be one- (1D),  two- (2D), or three-dimensional (3D). Generally, 1D systems are used for elevations, 2D for horizontal positioning, and both combined for 3D.  While small scale projects can use a local assumed system, those covering larger areas require more rigor as they must account for the Earth's shape and dynamics.

Most applications use a horizontal 2D grid coordinate system with elevations added if relief information is needed. This is largely because of:

• Surveying measurements - Horizontal (traversing) and vertical (leveling) measurements are done separately using different equipment and procedures.
• Mapping - Referenced to a horizontal base with separate depiction of elevations (eg, contours, TINs, DEMs, etc), even in digital environments.
• Specific characteristics - Horizontal and vertical positioning have unique issues that aren't resolved by combining them into a single 3D system