i. Relative GPS baseline standard errors can be obtained from the baseline reduction output and, in
some software programs, can be directly input into the adjustment. These standard errors, along with
their correlations, are given for each vector component (X, Y, and Z). They are converted to relative
weights in the adjustment. The following input weighting is commonly used:
Fixed. 3 millimeters (latitude) 5 millimeters (longitude) + 1 ppm 10 millimeters
(height) + 1 ppm.
Float. 6 millimeters (latitude) 10 millimeters (longitude) + 2 ppm 10 millimeters
(height) + 2 ppm.
These optimum standard errors have been found to be reasonable in standard work where extremely
long baselines are not involved. Use of these optimum values is recommended for the first adjustment
iteration.
j. Error ellipses or 3D error ellipsoids generated from the adjustment variance-covariance matrixes
for each adjusted point are also useful in depicting the relative positional accuracy. The scale of the
ellipse may be varied as a function of the 2D deviation. A 2.45 sigma (or 95 percent) probability ellipse
is usually selected for output. The size of the error ellipse's relative distance or the azimuth accuracy
estimate between two adjacent points is a direct function of the size of these positional ellipses.
5-53. Evaluation of Adjustment Results. Surveys should be classified based on their horizontal-point
closure ratio or vertical-elevation difference standard (Table 5-5, page 5-51).
a. The horizontal-point closure ratio is determined by dividing the linear-distance misclosure of the
survey into the overall circuit length of the traverse, loop, or network line or circuit. When independent
directions or angles are observed (such as a conventional survey [traverse or triangulation]), these
angular misclosures may be distributed before assessing positional misclosure. In cases where GPS
vectors are measured in geocentric coordinates, the 3D positional misclosure is assessed.
b. The vertical accuracy of a survey is determined by the elevation misclosure within a level section
or level loop. For conventional-differential or trigonometric leveling, section or loop misclosures (in
millimeters) should not exceed the limits shown in Table 5-5, page 5-52, where the line or circuit length
is measured in kilometers. Fourth-order accuracies are intended for construction grading work.
Procedural specifications or restrictions pertaining to vertical-control surveying methods or equipment
should not be over restrictive.
EN0593
5-50
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