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<br />( <br />- ---1 <br />, <br />I <br />~d. <br />~ <br />" <br />=------==:.. <br /> <br />recorded in the field. <br /> <br />" <br />I, <br /> <br />The more modern techniques utilize sonar sounding equipment <br />for defining the geometry under the water by recording channel measure- <br />ments on a digital tape and a strip chart as a boat moves across the <br />stream; the boat is kept on course by following a laser beam. The <br />resultant data is handled by computer. Aerial photography is being <br />used to develop topographic maps of the flood plain on either side of <br />the stream. Cross-section positions are established and data cards <br />are punched automatically by digitizers operating in conjunction with <br />plotting equipment. Aerial and ground surface photographs are used <br />to establish characteristics of bridges and for determining roughness <br />coefficients for the flow. <br />It is IOOre desirable to obtain topography from a single source <br />than to attempt to match up results of various surveys. With a <br />single source. all elevations will have a common datum. Therefore, <br />relative to each other. locations in the vertical are correct. Of <br />course, it is most desirable to reference all vertical control to <br />mean sea level. but sparse data often prevents it. <br />Another useful type of field data is observed water surface <br />profiles and corresponding discharges. These data are needed to <br />verify that the geometric model and n-values are correct. Without <br />field measured events for verification, the calculated water surface <br />profiles must be viewed with some uncertainty. <br /> <br />, <br />f <br /> <br />4.17 <br />