Global Positioning System Reference
In-Depth Information
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C
HAPTER 8
[Fi
[30
N
ETWORK ADJUSTMENTS
Lin
—
0.0
——
No
PgE
This chapter deals with minimal or inner constraint solutions for a polyhedron of
stations, i.e., the GPS vector network. The relative locations in such networks are
usually more accurate than the geocentric location of the polyhedron. Typically, the
relative position accuracy is derived from fixed carrier phase solutions, whereas the
geocentric location is obtained from point positioning with pseudoranges. Despite
the versatility of GPS, there are still many situations in which terrestrial observations
such as angles and distances measured with theodolite and electronic distance mea-
surement (EDM) are useful for supplementing GPS vectors. We discuss combination
solutions that use additional parameters, such as three rotations and a scale factor.
The rotation parameters can absorb rotational misalignment between the coordinate
system of the GPS ephemeris and the terrestrial coordinate system. If applied in local
networks, these rotation parameters may also be useful for absorbing a geoid slope if
geoid undulations are not available in the terrestrial system.
The 3D geodetic model discussed in Chapter 2 is the most natural one to be used
for these network adjustments. Since GPS gives accurate geodetic height differences,
the clear distinction between orthometric and geodetic (ellipsoidal) heights is always
important. This is particularly true when traditional leveling is replaced with GPS
height determination.
The chapter contains three examples of vector adjustments. While observing GPS
vector networks has become a routine occurrence, these examples have some “his-
toric value” as they helped establish GPS as a tool for accurate surveying. The Mont-
gomery County geodetic network densification demonstrated the utility of GPS to
densify classical first-order horizontal geodetic networks in terms of the achievable
accuracy and the high degree of flexibility in network design. The Stanford Linear
Collider (SLC) engineering survey pioneered in the sense that millimeter accuracy
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