method by which the misclosure is distributed among the intermediate points in the traverse is a function
of the adjustment weighting technique.
(4) In the case of a simple EDM traverse adjustment, the observed distances (or position
corrections) are weighted as a function of the segment length and the overall traverse length (compass
rule) or to the overall sum of the latitudes or departures (transit rule). Two-dimensional EDM distance
observations are not dependent on their direction (a distance's X and Y components are uncorrelated).
GPS baseline vector components (in X, Y, and Z) are correlated due to the geometry of the satellite
solution. Since satellite geometry is continuously changing, remeasured baselines will have different
correlations between the vector components. Such data are passed down from the baseline reduction
software for use in the adjustment.
(5) The magnitude of the misclosure of the GPS baseline vectors at the initial point provides an
estimate of the internal precision or geometric consistency of the loop (survey). When this misclosure is
divided by the overall length of the baselines, an internal relative-accuracy estimate results. This
misclosure ratio should not be less than the relative-distance accuracy classification intended for the
survey. For example, if the position misclosure of a GPS loop is 0.08 meter and the length of the loop is
8,000 meters, then the loop closure is 0.08 divided by 8,000, which equals 1:100,000.
(6) When an adjustment is performed, the individual corrections or adjustments made to each
baseline (residual errors) provide an accuracy assessment for each baseline segment. A least squares
adjustment can also provide relative-distance accuracy estimates for each line, based on standard error
propagation between adjusted points. This relative-distance accuracy estimate is most critical in
engineering and construction work and represents the primary basis for assessing the acceptability of a
survey.
b. External Adjustment. An external (or fully constrained) adjustment is the process used to best fit
the survey observations to the established reference system. The internal adjustment provides adjusted
positions relative to a single, often arbitrary, fixed point. Most conventional surveys or GPS-Ss are
connected between existing stations on some predefined reference network or datum. These fixed
stations may be existing project-control points (on NAD 27) or stations on the NGRS (NAD 83). In
locales outside the continental United States (OCONUS), other local or regional reference systems may
be used.
(1) A simple, conventional EDM traverse between two fixed stations best illustrates the process
by which comparable GPS baseline vectors are adjusted. Unlike a loop traverse, the azimuth and
position misclosures are not wholly dependent on the internal errors in the traverse--the fixed points and
their azimuth references are not absolute but contain relative inaccuracies with respect to one another.
(2) A GPS-S between the same two fixed points also contains a 3D position misclosure. Due to
positional uncertainties in the two fixed points, this misclosure
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