LEVEL ROD

The most often used level rod is the Philadelphia rod, as shown in figure 15-43. It is a graduated wooden rod, made of two sections, and can be extended from 7 feet to 13 feet.

Each foot is subdivided into hundredths of a foot. Instead of each hundredth of a foot being marked with a line or tick, the distance between alternate hundredths is painted black on a white background. Thus the distance between the colors: the top of the black is even values, the bottom of the black is odd values, the tenths are numbered in black, the feet in red.

Figure 15-43.-Philadelphia level rod with target.

Direct Reading

Direct readings are taken off a self-reading rod, held plumb on a point by a rodman. If you are working to tenths of a foot, it is relatively simple to read the foot mark below the cross hair and the tenth mark which is closest to the cross hair. But, working to the hundredths of a foot is more complicated. For example, suppose you are making a direct reading which comes out as 5.76 feet. On a Philadelphia rod, the graduation marks are 0.01 foot wide and are 0.01 foot apart. For a reading of 5.76 feet, there are three black graduations between the 5.70-foot mark and the 5.76-foot mark, as shown in figure 15-44. Since there are three graduations, the rod may be misread as 5.73 feet instead of 5.76 feet.

The 5-foot mark or the 6-foot mark does not show in figure 15-44. While sighting through the level instrument, you might not be able to see the foot

Figure 15-44.-Direct reading of 5.76 feet on a Philadelphia rod.

marks. When you cannot see the next lower foot mark through the level instrument, you signal or ask the rodman to raise for red. The rodman should slowly raise the rod until the next lower red number comes into view.

NOTE: The feet measurements on the Philadelphia rod are in red.

Target Reading

Conditions that hinder direct reading, such as poor visibility, long sights, and partially obstructed sights, as through brush or leaves, sometimes make it

Figure 15-45.-Target.

necessary to use targets. The target is also used to mark a rod reading when numerous points are set to the same elevation from one instrument setup.

Targets (fig. 15-45) for the Philadelphia rod are usually oval, with the long axis at right angles to the rod, and the quadrants of the target painted alternately red and white. The target is held in place by a C-clamp and a thumbscrew. A lever on the face of the target is used for fine adjustment of the target to the line of sight of the level. The targets have rectangular openings approximately the width of the rod and 0.15 feet high through which the face of the rod may be seen. A linear vernier scale is mounted on the edge of the opening with the zero on the horizontal line of the target for reading to thousandths of a foot. When the target is used, the rodman takes the rod reading.

When sighting through the level instrument, the levelman motions either up or down so that the rodman can place the horizontal separation of the target in line with the horizontal cross hair of the instrument. When the horizontal separation and the horizontal cross hair coincide, the levelman waves ALL RIGHT.

After the levelman signals the all right, the rodman tightens the target clamp. Then the rodman holds the rod on the point again to ensure the target has not slipped and "waves" the rod by pushing it about a foot away from and towards his body to see if the rod was initially held in an absolutely vertical position. The levelman should recheck the target reading. If the horizontal cross hairs do not coincide, the target must

Figure 15-46.-Zero elevation point. 15-26

be reset. The rodman reads the target to feet, tenths, and hundredths of the nearest foot gradation below the horizontal quadrant separation line of the target. Equipment Operator's seldom use the vernier scale in earthwork operation.