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<br />",. -.~ <br /> <br />fl.......... <br />t:'~l <br /> <br />o <br /> <br />[i <br />IJ <br />L:l <br /> <br />F] <br />~ <br /> <br />r".j <br />toJ <br />w <br /> <br />fl <br />f:::j <br /> <br />,..: <br /> <br />,- -~ <br /> <br />L" <br />,~ <br /> <br />When calculating a GPS position, vertical distance is the hardest to accurately <br /> <br />calculate due to the relative position of the satellites, errors introduced by the atmosphere, and <br /> <br />method by which the receiver calculates universal time. Because the errors in bathymetry <br /> <br />mapped by GPS and sonar represent the sum of the errors in the sonar and GPS data, it is very <br /> <br />important to use a very high quality GPS system. The GPS system used in this study was a <br /> <br />Javad Oddessy LIIL2 RTK GPS with Glonass and Multi-path reduction options turned on. <br /> <br />This system has a published vertical accuracy of 15mm +/- 1.5 ppm. As a quick test of the <br /> <br />accuracy of the system in the field, a single point was surveyed repeatedly at a rate of about <br /> <br />once per minute. The point was located approximately lkm from the base station and the <br /> <br />standard deviation of the elevations was 0.002 meters which suggests that the positions gained <br /> <br />from the GPS were good to +/- 5mm. <br /> <br />The sonar unit used was an ODOM HyrographicSystems, Hydrot~ac - Single <br /> <br />Frequency, Portable Survey Sounder. This unit used a 200kHZ frequency with a published <br /> <br />accuracy of lcm +/- 1% of depth and an output resolution of lcm. No detailed study was <br /> <br />made to verify the accuracy of the sonar unit due to time constraints and the difficulty. of <br /> <br />making such determinations in a river system. However, the Hydrotrac outputs depth in <br /> <br />realtime to a screen which is visible to the user and by watching the realtime output of data <br /> <br />when the boat was still or in a section of river with plainbed features, it was possible to <br /> <br />visually verify that the readings did not generally vary by more than lcm. <br /> <br />One of the greatest hindrances to using sonar to map the channel bottom is that there is <br /> <br />a minimum depth requirement. In order for the sonar to get a reading off the bottom of the <br /> <br />.; "- <br /> <br />channel, the transducer must have at least half a meter of water underneath it. The transducer <br /> <br />was located approximately 15cm underwater as to give room to roll and minimize air <br /> <br />entrainment under the transducer head making it difficult to gather bathymetric data in areas <br /> <br />shallower than 75cm. <br /> <br />15 <br />