STAR*NET V6 - Circe

Chapter 10 ADJUSTMENTS IN GRID COORDINATE SYSTEMS
10.1 Introduction
The surface of the earth is curved. However, for most surveying purposes, it is more
convenient to assume that the earth is flat, and to work with rectangular plane
coordinates, such as Northings and Eastings.
When surveys are performed in very small areas and using arbitrary coordinates, the
surveyor can make the assumption that he or she is working on a plane that is tangent to
the earth in the area of interest. Any errors resulting from this assumption are generally
acceptable. In this case, the surveyor can run STAR*NET adjustments using a “Local”
coordinate system.
However, for surveys performed over larger extents, and to higher accuracies, the
physical shape of the earth does become a factor. And, even for small local surveys, it is
often required that they be connected into a larger overall coordinate reference system
consistent for an entire city, county or region. The solution to this problem is to project
the curved surface of the earth onto another surface, such as a cylinder or a cone. The
new surface can then be laid flat onto a plane, and serve as the basis for map display and
computations. As long as the mathematical functions for transforming between the earth
surface and the flat plane are known, the surveyor can measure surface field data in the
curved earth system, and accurately represent it in a projected “Grid” coordinate system.
STAR*NET supports the following grid coordinate systems:
1. NAD27 and NAD83 – Defined grid zones used in the United States
2. UTM - Universe Transverse Mercator zones used domestically and internationally
3. Custom – Domestic and international user-defined grid systems
This chapter describes how STAR*NET works with grids, selecting of grid system zones
from the Project Options dialog, options relating to grid, output listing sections that
reflect the affects of the grid reductions, and setting up of custom grid systems.
The STAR*NET manual does not attempt to be a text book in this subject. If you plan to
run adjustments in grid coordinate systems, whether it be projections specific to the
United States, or other projections around the world, we suggest that you read some
technical information relating to these systems to get a general idea of the calculations
required for working these projections. There are excellent text books that go into great
detail explaining each system.
145