important in evaluating the overall adequacy of the observed network. Other statistics (such as chi-

square, confidence levels, or histograms) are usually not significant for lower-order engineering

projects, and become totally insignificant if the user is not well versed in statistics and adjustment

theory. Use of these statistics to reject data (or report the results of an adjustment) without

understanding their derivation and source within the network adjustment is not advised.

d. Relative-distance and positional accuracy estimates resulting from the free adjustment of a GPS

network are usually excellent in comparison to conventional surveying methods. Loop misclosures and

relative-distance accuracies between points commonly exceed 1:100,000.

e. Relative distance accuracy estimates between points in a network are determined by error

propagation in the positional standard errors at each end of the tie. Relative-accuracy estimates may be

derived for resultant distances or azimuths between the points. The relative-distance accuracy estimates

are those typically used to assess the free and constrained accuracy classifications, expressed as a ratio

(such as 1:80,000). Since each point in the network has particular position variances, the relative-

distance accuracy propagated between any two points will also vary throughout the network.

f. The minimum value (or the largest ratio) will govern the relative accuracy of the overall project.

This minimum value (from a free adjustment) is compared with the intended relative-accuracy

classification of the project to evaluate compliance. However, relative-distance accuracy estimates

should not be rigidly evaluated over short lines (less than 500 meters). Depending on the size and

complexity of the project, large variances in the propagated relative-distance accuracies can result.

When a constrained adjustment is performed, the adequacy of the external fixed stations will have a

major impact on the resultant, propagated distance-accuracies, especially when connections are made to

weak control systems. Properly weighted, partially constrained adjustments will usually improve the

propagated distance accuracies.

g. The primary criteria for assessing the adequacy of a particular GPS-S is based on the relative-

distance accuracy results from a minimally constrained free adjustment, not a fully constrained

adjustment. This is due to the difficulty in assessing the adequacy of the surrounding network. If the

propagated relative accuracies fall below the specified level, reobservation is warranted.

h. Most adjustment software will output the residual corrections to each observed baseline vector

component. These residuals indicate the amount by which each segment was corrected in the

adjustment. A least squares adjustment minimizes the sum of the squares of these baseline residual

corrections. Commercial least squares adjustment software packages are available. These software

packages adjust GPS networks using standard PCs. Sample adjustment statistics summaries from the

software package used by Army topographic surveyors are shown in Figure 5-8, page 5-50.

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