surveying iii (topographic and geodetic surveys) - Modern Prepper

i. Relative GPS baseline standard errors can be obtained from the baseline reduction output and, insome software programs, can be directly input into the adjustment. These standard errors, along withtheir correlations, are given for each vector component (X, Y, and Z). They are converted to relativeweights 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 longbaselines are not involved. Use of these optimum values is recommended for the first adjustmentiteration.j. Error ellipses or 3D error ellipsoids generated from the adjustment variance-covariance matrixesfor each adjusted point are also useful in depicting the relative positional accuracy. The scale of theellipse may be varied as a function of the 2D deviation. A 2.45 sigma (or 95 percent) probability ellipseis usually selected for output. The size of the error ellipse's relative distance or the azimuth accuracyestimate 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-pointclosure 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 thesurvey into the overall circuit length of the traverse, loop, or network line or circuit. When independentdirections or angles are observed (such as a conventional survey [traverse or triangulation]), theseangular misclosures may be distributed before assessing positional misclosure. In cases where GPSvectors 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 sectionor level loop. For conventional-differential or trigonometric leveling, section or loop misclosures (inmillimeters) should not exceed the limits shown in Table 5-5, page 5-52, where the line or circuit lengthis measured in kilometers. Fourth-order accuracies are intended for construction grading work.Procedural specifications or restrictions pertaining to vertical-control surveying methods or equipmentshould not be over restrictive.EN0593 5-50