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DISCHARGE MEASUREMENTS AT GAGING STATIONS Figure 24.-Connectors: fop left, Au connector with plastic sheave; top right, Au connector with metal sheave; middle, type-B connector; and bottom, pressed-sleeve connector. Figure 25.-Computing depth indicator. recorded is that to the streambed. The trans- ducer has a narrow beam angle of 60 which minimizes errors on inclined streambeds and allows the hydrographer to work close to piers or other obstructions. ;;:;l6-:?~ O-Gl)--- --I 17 Measurements can be made with this eqnip- ment without lowering the meter and weight to the streambed. As soon as the weight is in the water, the depth will be recorded. The meter can then be set at the 0.2 depth (see p. 31) or just below the water surface (see p.37) where a velocity reading is obtained. Then a coefficient is applied to convert measured velocity to the mean in the vertical. (See p.37.) Temperature change affects the sound propa- gation velocity, but this error is limited to about ::l: 2 percent in fresh water. This error can be eliminated completely by adjusting the sounder to read correctly at a particular average depth determined by other means. Width-measuring equipment The distance to any point in a cross section is measured from an initial point on the bank. Cableways and bridges used regularly for making discharge measurements are commonly marked at 2-, 5-, to-, or 20-foot intervals by paint marks. Distance between markings is estimated, or measured with a rule or pocket tape. For measurements made by wading, from boats, or from unmarked bridges, steel or metallic tapes or tag lines are used. Tag lines are made of y,,-, ){,-, %,-, or X-inch diameter galvanized steel aircraft cord with solder beads at measured intervals to indicate distances. The standard arrangement of solder beads or tags is: - Number of tag, (Jut) ArraftlflfMnl or atation 1_______________ 2 0-50 1_______________ 5 50-150 L_____h_______ 10 ISO to end 2__h__~________________ 0,10,20,30,40,50, 150, 250, 350, 450 3_______________________ 100,200,300,400,500 The standard lengths of tag line are 300, 400, and 500 feet, but other sizes are available. Three types of tag-line reels in use (fig. 33) are Lee-Au, Pakron, and Columbus type A. Larger reels designed particularly for use with boats are described on page 24. It is practically impossible to string a tag line for discharge measurements from a boat when the width of the stream is greater than 2,500 feet. The methods used to determine width at such places are described on page 44.