The three most common situations in which target readings rather than direct readings are made are as follows: (1) when the rod is too far from the instrument to be read directly; (2) when you desire a reading to the nearest 0.001 ft, which requires the use of the vernier by the rodman; and (3) when the instrumentman thinks a reading by the rodman instead of by himself more likely will be accurate.

For target readings up to 7.000 ft, the Philadelphia rod is used fully closed and read on the face by the rodman. The rodman sets the target on the face by the signals from the instrumentman, who determines when the horizontal axis of the target intercepts the horizontal cross hair.

When the instrumentman signals "all right," the rodman clamps the target in place with the target screw clamp, as shown in figure 14-9; then

Figure 14-10.-Reading of 7.107 ft on back of Philadelphia rod as indicated by arrow.

the rodman reads the target vernier, shown in the same figure.

The reading to the nearest 0.01 ft is indicated by the zero on the vernier. In figure 14-9, the vernier zero indicates a reading of a few thousandths of a foot more than 5.84 ft. To determine how many thousandths over 5.84 ft, you examine the graduations on the vernier to determine the one most exactly in line with a graduation on the rod. In figure 14-9, this is the 0.003-ft graduation; therefore, the reading to the nearest 0.001 ft is 5.843 ft.

For target readings of more than 7.000 ft, the Philadelphia rod is used extended; the rodman makes the reading on the back of the rod. In figure 14-10, the back of the upper section of the rod is shown, graduated FROM THE TOP DOWN, from 7.000 ft through 13.000 ft. You can also see a rod vernier on the back, fixed to the top of the lower section of the rod, also reading from the top down.

For a target reading of more than 7.000 ft, the rodman, on receiving the signal to "extend the rod," fixes the target on the face of the upper section all the way to the top of the upper section. While doing this, the rodman makes sure the target vernier is set at exactly the same reading indicated by the rod vernier on the back of the rod, He then releases the rod screw clamp and slides the upper section of the rod slowly upwards until the instrumentman gives the signal "all right." When the horizontal axis of the target reaches the level where it is intersected by the horizontal cross hair, the instrumentman gives this signal.