Introduction: What is Geodetic Control?
Geodetic control survey points support the production of consistent and spatial accurate data for surveying and mapping. Permanent geodetic monuments provide a consistent coordinate system and serve as the basis for computing lengths and distances between relative positions. To this end, the National Spatial Reference System, managed by the Federal Geodetic Control Subcommittee
(FGCS) of the National Geodetic Survey
(NGS), is the standard for defining horizontal and vertical position, gravity, and orientation of control points in the United States.
Types of Geodetic Control
There are two different types of geodetic control: horizontal and vertical. Horizontal control points are typically placed at higher locations to create a clear line of sight and are measured by their position on the earth's ellipsoid, a mathematical surface used to approximate the shape of the Earth. Vertical control points provide elevations with reference to the geoid, an irregular surface of constant gravitational force.
The vertical control network is comprised of monumented stations on which precise elevations have been determined. Vertical control points are typically spaced in close proximity to one another, typically along railroads or highways. Differential leveling is the most common method of determining elevation. In differential leveling, a sequence of lines-of-sight is established. Two readings are taken along the line of sight, one at a known benchmark elevation and the other at a point of unknown elevation. The difference between the two is used to establish the elevation of the unknown point. For geodetic work, three-wire leveling is used. In this method, a leveling instrument with an eyepiece having three separate horizontal lines is used. Elevation is determined by finding the average reading for each of the horizontal lines. GPS can also be used to obtain vertical heights, but this method is inaccurate due to limited knowledge of the geoid.
The horizontal control network is made up of monumented stations on which precise latitude and longitude have been established. Horizontal control can be established by a number of methods, the most common being triangulation and trilateration. Using triangulation, the location of survey points is calculated using the measurement of angles in a network of triangles. Lines of sight are obtained by placing the stations on high points. Angles are then measured to other distant points and networks of connected triangles are formed. A similar method of creating networks of connected triangles is trilateration. Distances between stations, rather than angles, are measured. A third method of establishing horizontal control is GPS, which avoids line-of-sight problems and is faster, cheaper, and more accurate than traditional surveying methods.
Producers of Geodetic Control
Surveys that follow guidelines for standards and specifications must be submitted to the NGS for analysis in order to become part of the NSRS. If accepted, the data is added to the NSRS database.