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.
EN0593
5-48
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