# Positioning Techniques for Offshore Engineering Surveys

## Quiz Questions

 1. Terrestrial positioning methods include traditional land-based techniques such as Sextant resection Triangulation Tag lines Microwave electronic distance measurement (EDM) systems Electronic total stations All of the above None of the above 2. Sextant positioning is not used to calibrate medium frequency hyperbolic, range-range, and microwave positioning systems. True False 3. Inexpensive hand-held GPS (Standard Positioning Service–SPS) receivers typically provide accuracies that equal or exceed sextant positioning. True False 4. The US Coast Guard used the __________________ technique on buoy tenders. Hopper dredge positioning Redundant sextant re-sectioning 5. Plotting sextant fixes with a three-arm protractor aboard a moving vessel is not an exact process, and significant inaccuracies can result. True False 6. In equation 7-1, σ = (δA + δB) / 2, What does δA represent? δA = d A-P . sin (δ a “ ) δA= d B-P . sin (δ a “ ) 7. Multiple azimuth intersection techniques provide redundancy and allows for an on-line assessment of the accuracy of the resultant position. True False 8. Visual range-pair alignment accuracy is a function of the _______________ from the targets and the range pair spread. Distance Displacement 9. The most ___________________tag line distance measurements are conducted while the survey boat is stationary and holding constant tag line tension and alignment. Accurate Inaccurate 10. Lateral alignment control of the survey boat can be the __________________ link in the performance of tag line surveys, especially if strong currents are present. Weakest Strongest 11. Wire rope tag line lengths vary from__________________ and baseline boat tag lines from ____________________ long. 100 ft to over 1,000 ft, 1,000 ft to 10,000 ft 500 ft to over 5,000 ft, 5,000 ft to 15,000 ft 1000 ft to over 5,000 ft, 5,000 ft to 15,000 ft 12. The tag line cable is marked at any desired interval, usually every 25 ft. True False 13. ______________swivels are inserted along the tag line at intermediate points, usually at the 100-ft mark, the 500-ft mark, and at subsequent 500-ft intervals thereafter. Corrosion-resistant Corrosion-irresistant 14. Clutching and braking assemblies in the power winches __________________tag line payout. Regulate Do not regulate 15. Tag line surveys are highly accurate only within finite limits. True False 16. Flagged tag line intervals must be periodically calibrated every __________________against a chained or EDM distance. 1 to 3 months 3 to 6 months 6 to 9 months 17. The accuracy of a range-azimuth position can be estimated from the following equation: RMS 95% = 1.73 • sqrt [ a 2 + ( d • tan b) 2 ] where ‘a’ is ______________________. Estimated standard error of azimuth measuring system Estimated standard error of the distance measuring system 18. EDM is an acronym for ________________________. Electronic distance measurement Electronic displacement measurement 19. Land-based (or terrestrial) positioning systems are not distinguished from satellite positioning systems. True False 20. The method of classifying electronic positioning systems is by their operating ________________. Frequencies Pulse Voltage 21. LORAN-C is a _______________________ time-differencing hyperbolic system. Low frequency High frequency 22. Variations in the velocity of propagation in air are caused by changes in air density due to temperature, humidity, and air pressure. True False 23. Signal multipath reflection is not a major systematic error component for equipment operating in the microwave band. True False 24. In equation 7-7, an approximate estimate of the range accuracy is obtained as : Estimated Range Accuracy (1- σ ) = sqrt [ Σ ( v2 ) / (n-1) ], where Σ ( v2) is __________________. Sum of the observed ranges Sum of the squared residuals 25. _______________ Calibrations are performed by locating the survey vessel alongside a known reference point and comparing the computed (inversed) distances with the ranges observed by the microwave system. EDM Baseline