ground. Make any slight adjustments to level the headfurther by moving the third leg a few inches in or outbefore pressing it into the ground.When placing the instrument onto the tripod, gripthe instrument firmly to avoid dropping it while youare mounting it on the tripod. The instrument shouldbe screwed down to a firm bearing, but not so tightlythat it binds or the screw threads strip.LEVEL RODThe most often used level rod is the Philadelphiarod, as shown in figure 15-43. It is a graduatedwooden rod, made of two sections, and can beextended from 7 feet to 13 feet.Each foot is subdivided into hundredths of a foot.Instead of each hundredth of a foot being marked witha line or tick, the distance between alternatehundredths is painted black on a white background.Thus the distance between the colors: the top of theblack is even values, the bottom of the black is oddvalues, the tenths are numbered in black, the feet inred.Figure 15-43.—Philadelphia level rod with target.Direct ReadingDirect readings are taken off a self-reading rod,held plumb on a point by a rodman. If you are workingto tenths of a foot, it is relatively simple to read thefoot mark below the cross hair and the tenth markwhich is closest to the cross hair. But, working to thehundredths of a foot is more complicated. Forexample, suppose you are making a direct readingwhich comes out as 5.76 feet. On a Philadelphia rod,the graduation marks are 0.01 foot wide and are 0.01foot apart. For a reading of 5.76 feet, there are threeblack graduations between the 5.70-foot mark and the5.76-foot mark, as shown in figure 15-44. Since thereare three graduations, the rod may be misread as 5.73feet instead of 5.76 feet.The 5-foot mark or the 6-foot mark does not showin figure 15-44. While sighting through the levelinstrument, you might not be able to see the footFigure 15-44.—Direct reading of 5.76 feet on a Philadelphiarod.15-25
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