TURNING POINT.- The two missing grade stake examples were based on elevations of nearby points that could be read from one setup of the level. If differences of elevation or distance are too great or if there are obstructions, you will have to make an intermediate setup and sight on a point, called a turning point (T.P.). Any convenient point may be used as a turning point, but the level rod must be set on firm ground or on some firm object so that the elevation of the T.P. will not change while the rodman waits for the levelman to setup at the new position.

An example of a level run is shown in figure 15-54. You have a B.M. at the bottom of a bank and you want to find the elevation at the top of the bank, which is point A. You cannot set up on the top of the bank to take a reading on the level rod held on point A because to take a backsight shot on the B.M., the level rod is too short.

First, record the B.M. elevation of 120.0 feet. Next, set up the level instrument on the bank and take a backsight shot on the B.M. to get a level rod reading of 10.2 feet. Add the backsight shot of 10.2 feet to the B.M. elevation of 120.0 feet to get the first instrument height of 130.2 feet. Then take a foresight shot on the T.P. to get a level rod reading of 1.2 feet. Subtract the 1.2 feet foresight reading from the 130.2 feet instrument height to get a T. P. elevation of 129.0 feet.

Next, move the instrument to the top of the bank. Take a backsight shot on the T.P. to get a level rod reading of 9.8 feet. Add the 9.8 feet backsight reading to the 129.0 feet T.P. elevation to get a second instrument height of 138.8 feet. The last step is to take a foresight shot on point A to get a level rod reading of 3.8 feet. Subtract the 3.8 feet foresight reading from the 138.8 feet second instrument height to get a 135.0 feet point A elevation.

Some level runs may require more than one T. P.; however, no matter how extensive the job, the procedure is always the same: you add and subtract successive rod readings from a point of known elevation to the point of unknown elevation.

MEASURING HORIZONTAL DISTANCES

Setting or replacing grade stakes requires measuring horizontal distances with either a woven tape or a steel tape.

Figure 15-54.-Turning point and level notes. 15-32

A woven tape (fig. 15-55 ) is made of high-grade cloth (usually linen) fabric. A metallic, woven tape is reinforced with fine bronze or brass wire mesh. A nonmetallic, woven tape does not contain the mesh; however, some nonmetallic, woven tapes are coated with plastic.

Woven tapes are made in 25-, 50-, 75-, 100-, and 150-foot lengths. Some are graduated in feet and inches to the nearest quarter inch. Others are graduated in feet and decimals of a foot to the nearest 0.05 foot. On most decimally graduated woven tapes, only the 0.10-foot graduations are marked with numerals.

The steel tape is used for measurements requiring greater precision than is possible with the woven tape. The most commonly used steel tape is 100 feet in length and is graduated in feet, tenths, and hundredths. Some steel tapes are graduated throughout; on others, only the first foot is graduated in subdivisions and the body of the tape is graduated only at every 1-foot mark. A steel tape is sometimes equipped with a reel on which the tape can be wound. The tape can be detached from the reel for more convenient use in taping.

For convenience in carrying from one place to another, a detached tape can be made up into a coil, commonly called "DOING UP' the tape. This is done by placing the 100-foot end (or the 200-foot, 300-foot, etc., end) in your left hand, faceup; then reach back with your right hand, grasp the 95-foot mark, bring it up, and place it faceup on top of the 100-foot mark. Do the same

Figure 15-55.-Woven tape.

with the 90-foot mark, the 85-foot mark, the 70-foot mark, and so forth, until you have gathered in the entire tape. You will find that the tape now forms a figure-ofeight, as shown in figure 15-56. The figure-of-eight can be formed into a circular coil, as shown in figure 15-57.

Figure 15-56.-Doing up a steel tape.

Figure 15-57.-Steps in throwing a steel tape into a coil.