Laserfiche WebLink
<br />the precision to which angles can be measured and distances can be measured. Because <br /> <br />distance can be measured repeatedly and can be averaged, accuracy is generally contingent <br /> <br />upon angle measurement and can be expressed as a function of the measurement distance. <br /> <br />With a horizontal aI)d vertical accuracy of 3 arc seconds, the relative horizontal and vertical <br /> <br />error is 1.45E-5 of the measurement distance. Because an effort was made to not shoot points <br /> <br />at a distance of more than 100Dft from the total station, the amount of error introduced by the <br /> <br />machine was limited to less than +/- 4mm. <br /> <br />Global Positioning Systems.and Sonar <br /> <br />In 1999, a second technique was used to gather bathymetric surveys of the channel. <br /> <br />Global Positioning Systems (GPS) and sonar technology were used together to allow the <br /> <br />mapping of the channel from a moving boat, thereby gathering a large amount of data in a <br /> <br />short amount of time. In recent years there have been advances in GPS technology but the <br /> <br />basic concept ofGPS remains the same. GPS satellites with known orbits broadcast pseudo <br /> <br />random code which is synchronized to universal time. A GPS receiver receives the signal and <br /> <br />calculates a distance to the satellites based on the amount of time required for the signal to <br /> <br />reach the receiver. Using trigonometry, it is possible to locate a single point in space if you <br /> <br />know the distance to four other known points. <br /> <br />The GPS satellites have atomic clocks on board which allow them to all broadcast <br /> <br />pseudo random code in complete synchronicity. Because the receiver does nothave an atomic <br /> <br />clock on board from which to determine universal time, it looks for a single time correction <br /> <br />......--'.. ~ <br />",""" .~ .,'"- <br />that will allow the receiver to resolve the four time signals to into a single point. Once the <br /> <br />receiver has determined universal time, it can then determine the time offset and distance to <br /> <br />the satellites. <br /> <br />14 <br />