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<br />o <br /> <br />field-surveyed cross section points shall be . <br />within ~0.5 foot of true elevations. <br /> <br />Cross sections shall be located perpendicular <br />to the direction of flow at appreciable <br />changes in flow area. roughness. or stream <br />gradient. Additional cross sections shall be <br />located at bridges. at the head and tail of <br />levees. at confluences with tributaries. and <br />at all flow control structures. In general. <br />more cross sections are needed to define <br />energy losses in urban areas than in' rural <br />areas. where steeper slopes are encountered. <br />and on smaller streams. Based on computed <br />results. additional cross sections may be <br />required if the slope of the energy grade line <br />between successive cross sections decreases by <br />more than 50 percent or increases by more than <br />100 percent. The location of the cross- <br />sections shall be shown on the work maps. <br /> <br />8. Map Accuracy. Vertical map accuracy shall <br />be such that at least 90 percent of all <br />contours are wi thin 1/2 of the contour <br />interval. and the remaining contours are not <br />in error by more than 1 contour interval. <br />Horizontal map accuracy shall be such that at . <br />least 90 percent of the planimetric features <br />are plotted to within 1/40 inch as measured at <br />the map scale. and the remaining features are <br />plotted to within 1/20 inch. <br /> <br />In areas where the ground is completely <br />obscured by dense brush or tree cover. <br />contours shall be plotted as accurately as <br />possible from the stereoscopic model. while <br />making full use of spot elevations obtained <br />where the ground is visible. Spot elevations <br />shall have the same vertical and horizontal <br />tolerance as the contours. Where appl icable. <br />the work map shall carry the statement: "This <br />map complies with National Map Accuracy <br />Standards." <br /> <br />B. Hydrologic Analysis <br /> <br />1. Frequency. The hydrologic analysis sha 11 <br />include calculations for the~. ..-!1(1'-. 100-. <br />aR4 ~99-year frequency flood peak discharges. <br /> <br />2. MethodoloQY. Peak discharges developed from <br />hydrologic analysis for floodplain management <br />purposes shall be determined by one or more of <br /> <br />. <br /> <br />6 <br />