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

Integrated Publishing, Inc. |