not be designed or performed to achieve accuracy levels which exceed the project requirements. For
instance, if a third-order, Class I accuracy standard (1:10,000) is required for most of the topographic-
survey control on a project, field survey criteria should also be designed to meet this minimum standard.
5-31. Network Design Factors. Some of the factors to be considered in designing a GPS network and
subsequent observing procedures are the--
a. Project Size. The extent of the project will affect the GPS-S network shape.
b. Required Density of Control. The type of GPS-S scheme used will depend on the number and
spacing of points to be established, which is a project-specific requirement. Maximum baseline lengths
between stations or existing control are also prescribed. A combination of GPS and conventional-survey
densification is often the most effective approach.
c. Absolute GPS Reference Datums. Coordinate data for baseline observations are referenced and
reduced relative to the WGS-84 earth-centered earth-fixed (ECEF) coordinate system (X, Y, and Z). For
all practical purposes, this system is not directly referenced to (though closely related) the Geodetic
Reference System of 1980 (GRS 80) upon which NAD 83 is related (for CONUS work). Data reduction
and adjustment are normally performed using the WGS-84 ECEF coordinate system, with baseline
vector components measured relative to the ECEF coordinate system. The baseline vector components
are denoted by delta (Δ) X, ΔY, and ΔZ.
d. Connections to Existing Control. For most static and kinematic GPS horizontal-control work, at
least two existing control points should be connected for referencing and adjusting a new GPS-S. Table
5-3 shows GPS-S design, geometry, connection, and observing criteria. Existing points may be part of
the NGRS or in place project control that has been adequately used for years. Additional points may be
connected if practical. In some instances, a single existing point may be used to generate spurred
baseline vectors for supplemental construction control.
EN0593
5-16
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