necessary to determine approximate orthometric elevations from relative GPS observations.
a. The baseline reduction process provides the raw relative position coordinates that are used in a
3D GPS network adjustment. Depending on the manufacturer's software, each reduced baseline
contains various orientation parameters, covariance matrices, and cofactor or correlation statistics that
may be used in weighting the final network adjustment. Most least squares adjustments use the
accuracy or correlation statistics from the baseline reduction; however, other weighting methods may be
used in a least squares or approximate adjustment.
b. The adjustment technique employed (and the time devoted to it) must be commensurate with the
project's accuracy requirements. Care must be taken to prevent the adjustment process from becoming a
project in itself.
c. There is no specific requirement that a rigorous least squares adjustment be performed on
topographic surveys, whether conventional, GPS, or mixed observations. Traditional approximate-
adjustment methods may be used in lieu of the least squares method and provide comparable, practical
accuracy results.
d. Commercial software packages designed for higher-order geodetic-densification surveys often
contain a degree of statistical sophistication that is unnecessary for engineering survey control
densification (such as second-order or less). The distinction between geodetic surveying and
engineering surveying must be fully considered when performing GPS-S adjustments and analyzing the
results.
e. Connections and adjustments to existing control networks, such as the National Geodetic
Reference System (NGRS), must not become independent projects. It is far more important to establish
dense and accurate local-project control than to consume resources tying into first-order NGRS points
that are miles from the project. Engineering, artillery, construction, and property/boundary referencing
requires consistent local control with high relative accuracies. Accurate connections and references to
distant geodetic datums are of secondary importance (the exception being projects in support of military
aviation operations). The advent of GPS-S technology has provided a cost-effective means of tying
previously poorly connected projects to the NGRS and simultaneously transforming the projects to the
newly defined NAD 83. When performing (adjusting) these connections, do not distort or warp long-
established project reference points.
5-48. Survey Accuracy. The accuracy of a survey (whether performed using conventional or GPS
methods) is a measure of the difference between observed and true values (such as coordinates, distance,
or angle). Since the true values are rarely known, only estimates of survey accuracy can be made.
These estimates may be based on internal observation closures (such as on a loop traverse) or
connections with previously surveyed points assumed to have some degree of reliability.
a. A loop traverse originating and ending from a single point will have a misclosure when
observations (such as EDM traverse angles or distances or GPS
EN0593
5-42
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