DIRECT READINGS

As the levelman, you can make direct readings on a self-reading rod held plumb on the point by the rodman. If you are working to tenths of a foot, it is relatively simple to read the footmark below the cross hair and the tenth mark that is closest to the cross hair. If greater precision is required, and you must work to hundredths, the reading is more complicated (see figure 5-10).

For example, suppose you are making a direct reading that should come out to 5.67 feet. If you are using a Philadelphia rod, the interval between the top and the bottom of each black graduation and the interval between the black graduations (figure 5-11 ) each represent 0.01 foot. For a reading of 5.76 feet, there are three black graduations between the 5.70-foot mark and the 5.76-foot mark. Since there are three graduations, a beginner may have a tendency to misread 5.76 feet as 5.73 feet.

As you can see, neither the 5-foot mark nor the 6-foot mark is shown in figure 5-11. Sighting through the telescope, you might not be able to see the foot marks to which you must refer for the reading. When you cannot see the next lower foot mark through the telescope, it is a good idea to order the rodman to raise the red. On the Philadelphia rod, whole feet numerals are in red. Upon hearing this order, the rodman slowly raises the rod until the next lower red figure comes into view.

TARGET READINGS

For more precise vertical measurements, level rods may be equipped with a rod target that can be set and clamped by the rodman at the directions of the instrumentman. When the engineer's level rod target and the vernier scale are being used, it is possible to make readings of 0.001 (one-thousandth of a foot), which is slightly smaller than one sixty-fourth of an

Figure 5-10.-Philadelphia rod marking.

Figure 5-11.-Direct reading of 5.76 ft on Philadelphia rod.

inch. The indicated reading of the target can be read either by the rodman or the instrumentman. In figure 5-12, you can see that the 0 on the vernier scale is in exact alignment with the 4-foot mark. If the position of the 0 on the target is not in exact alignment with a line on the rod, go up the vernier scale on the target to the line that is in exact alignment with the hundredths line on the rod, and the number located will be the reading in thousandths.

Figure 5-12.-Target.

There are three situations in which target reading, rather than direct reading, is done on the face of the rod:

When the rod is too far from the level to be read directly through the telescope:

When a reading to the nearest 0.001 foot, rather than to the nearest 0.01 foot, is desired (a vernier on the target or on the back of the rod makes this possible;

When the instrumentman desires to ensure against the possibility of reading the wrong foot (large red letter) designation on the rod.

For target readings up to 7.000 feet, the rod is used fully closed, and the rodman, on signals from the instrumentman, sets the target at the point where its horizontal axis is intercepted by the cross hair, as seen through the telescope. When the target is located, it is clamped in place with the target screw clamp, as shown in figure 5-9. When a reading to only the nearest 0.01 foot is desired, the graduation indicated by the target's horizontal axis is read; in figure 5-9, this reading is 5.84 feet.

If reading to the nearest 0.001 foot is desired, the rodman reads the vernier (small scale running from 0 to 10) on the target. The 0 on the vernier indicates that the reading lies between 5.840 feet and 5.850 feet. To determine how many thousandths of a foot over 5.840 feet, you examine the graduations on the vernier to determine which one is most exactly in line with a graduation (top or bottom of a black dash) on the rod. In figure 5-9, this graduation on the vernier is the 3; therefore, the reading to the nearest 0.001 foot is 5.843 feet.

For target readings of more than 7.000 feet, the procedure is a little different. If you look at the left-hand view of figure 5-8 (showing the back of the rod), you will see that only the back of the upper section is graduated, and that it is graduated downward from 7.000 feet at the top to 13.09 feet at the bottom. You can also see there is a rod vernier fixed to the top of the lower section of the rod. This vernier is read against the graduations on the back of the upper section.

For a target reading of more than 7.000 feet, the rodman first clamps the target at the upper section of the rod. Then, on signals from the instrumentman, the rodman extends the rod upward to the point where the horizontal axis of the target is intercepted by the cross hair. The rodman then clamps the rod, using the rod clamp screw shown in figure 5-13, and reads the vernier on the back of the rod, also shown in that figure. In this case, the 0 on the vernier indicates a certain number of thousandths more than 7.100 feet. Remember, that in this case, you read the rod and the vernier down from the top, not up from the bottom. To determine the thousandths, determine which vernier graduation lines up most exactly with a graduation on the rod.  In this case, it is the 7; therefore, the rod reading is 7.107 feet.

Rod Levels

A rod reading is accurate only if the rod is perfectly plumb (vertical) at the time of the reading. If the rod is out of plumb, the reading will be greater than the actual vertical distance between the height of

Figure 5-13.-Philadelphia rod target reading of more than 7.000 ft.

instrument (H.I.) and the base of the rod. On a windy day, the rodman may have difficulty holding the rod plumb. In this case, the levelman can have the rodman wave the rod back and forth, allowing the levelman to read the lowest reading touched on the engineer's level cross hairs.

The use of a rod level ensures a vertical rod. A bull's-eye rod level is shown in figure 5-14. When it is held as shown (on a part of the rod where readings are not being taken to avoid interference with the instrumentman's view of the scale) and the bubble is centered, the rod is plumb. A vial rod level has two spirit vials, each of which is mounted on the upper edge of one of a pair of hinged metal leaves. The vial level is used like the bull's-eye level, except that two bubbles must be watched instead of one.

Care of Leveling Rods

A leveling rod is a precision instrument and must be treated as such. Most rods are made of carefully selected, kiln-dried, well-seasoned hardwood. Scale graduations and numerals on some are painted directly on the wood; however, on most reds they are painted on a metal strip attached to the wood. Unless a rod is handled at all times with great care, the painted scale will soon become scratched, dented, worn, or otherwise marked and obscured. Accurate readings on a scale in this condition are difficult.

Allowing an extended sliding-section rod to close on the run, by permitting the upper section to drop, may jar the vernier scale out of position or otherwise damage the rod.             Always close an extended rod by easing the upper section down gradually.

A rod will read accurately only if it is perfectly straight. It follows, then, that anything that might bend or warp the rod must be avoided. Do not lay a rod down flat unless it is supported throughout, and never use a rod for a seat, a lever, or a pole vault. In short, never use a rod for any purpose except the one for which it is designed.

Store a rod not in use in a dry place to avoid warping and swelling caused by dampness. AI ways wipe off a wet rod before putting it away. If there is dirt on the rod, rinse it off, but do not scrub it off. If a soap solution must be used (to remove grease, for example), make it a very mild one. The use of a strong soap solution will soon cause the paint on the rod to degenerate.

Figure 5-14.-Bull's-eye rod level.

Protect a rod as much as possible against prolonged exposure to strong sunlight. Such exposure causes paint to chalk (that is, degenerate into a chalk-like substance that flakes from the surface).

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