may (and usually does) far exceed the internal accuracy of the raw GPS observations. As with a
conventional EDM traverse, the 3D misclosures may be about adjusted by proportionately distributing
them over the intermediate points. A least squares adjustment also accomplishes the same thing. For
example, if the GPS-S is looped back to the initial point, the free-adjustment misclosure at the initial
point may be compared with the apparent position misclosure with the other fixed point. A free-
adjustment loop misclosure is 1:100,000, whereas the misclosure relative to the two network control
points is only 1:5,000. Thus, the internal relative accuracy of the GPS-S is about 1:100,000 (based on
the misclosure). If the GPS-baseline observations are constrained to fit the existing control, the 0.6-
meter external misclosure must be distributed among the individual baselines to force a fit between the
two end points.
If the intent of the above example was to establish 1:20,000 relative-accuracy control,
connecting between these two points will not provide that accuracy given the amount of adjustment that
must be applied to force a fit. For example, if one of the individual baseline vectors was measured at
600 meters and the constrained adjustment applied a 0.09-meter correction in this sector, the relative
accuracy of this segment would be roughly 1:6,666. This distortion is not acceptable for subsequent
design/construction work.
(3) Most GPS-S networks are more complex than a simple traverse. They may consist of
multiple loops and may connect with any number of control points on the existing network.
Additionally, conventional EDM, angles, and differential-leveling measurements may be included with
the GPS baselines, resulting in a complex network with many adjustment conditions.
5-51. Partially Constrained Adjustments. In the previous example of a simple GPS traverse, holding
the two network points rigidly caused an adverse degradation in the GPS-S because of the differences
between the free (loop) adjustment and the fully constrained adjustment. Another alternative is to
perform a partially constrained adjustment of the net. In a partially constrained adjustment, the two
network points are not rigidly fixed but are only partially fixed in position. Partially constrained
adjustments are not practicable using approximate adjustment techniques.
a. For example, if the relative-distance accuracy between the two fixed points is about 1:10,000, it
can be equated to a positional uncertainty between these points. Depending on the type and capabilities
of the least squares adjustment software, the higher-accuracy GPS baseline observations can be best fit
between the two end points. The end points of the GPS network are not rigidly constrained to the
original and two control points but will end up falling near them.
b. Adjustment software allows relative weighting of the fixed points to provide a partially
constrained adjustment. Any number of fixed points can be connected, and these points may be given
partial constraints in the adjustment.
EN0593
5-46
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