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Leveling—Field Procedures and Computations

Carrying and Setting Up a Level

The safest way to transport a leveling instrument in a vehicle is to leave it in the container. The case closes properly only when the instrument is set correctly in the padded supports.A level should be removed from its container by lifting from the base, not by grasping the telescope. The head must be screwed snugly on the tripod. If the head is too loose, the instrument is unstable; if too tight, it may “freeze.” Once the instrument is removed from the container, the container should be once again closed to prevent dirt and moisture from entering it.

The legs of a tripod must be tightened correctly. If each leg falls slowly of its own weight after being placed in a horizontal position, it is adjusted properly. Clamping them too tightly strains the plate and screws. If the legs are loose, unstable setups result.

Except for a few instruments that employ a ball-and-socket arrangement, all modern levels use a three-screw leveling head for initial rough leveling. Note that each of the levels illustrated in Chapter 4 (see Figures 4.9, 4.13, 4.14, and 4.16) has this type of arrangement. In leveling a three-screw head, the telescope is rotated until it is over two screws as in the direction AB of Figure 5.1. Using the thumb and first finger of each hand to adjust simultaneously the opposite screws approximately centers the bubble. This procedure is repeated with the telescope rotated 90° so that it is over C, the remaining single screw. Time is wasted by centering the bubble exactly on the first try, since it can be thrown off during the cross leveling. Working with the same screws in succession about three times should complete the job. A simple but useful rule in centering a bubble, illustrated in Figure 5.1, is: A bubble follows the left thumb when turning the screws. A circular bubble is centered by alternately turning one screw and then the other two. The telescope need not be rotated during the process.

It is generally unnecessary to set up a level over any particular point.Therefore it is inexcusable to have the base plate badly out of level before using the leveling screws. On sidehill setups, placing one leg on the uphill side and two on the downhill slope eases the problem. On very steep slopes, some instrument operators prefer two legs uphill and one downhill for stability. The most convenient height of setup is one that enables the observer to sight through the telescope without stooping or stretching.

Inexperienced instrument operators running levels up or down steep hillsides are likely to find, after completing the leveling process, that the telescope is too low for sighting the upper turning point or benchmark. To avoid this, a hand level can be used to check for proper height of the setup before leveling the instrument precisely. As another alternative, the instrument can be quickly set up without attempting to level it carefully. Then the rod is sighted making sure the bubble is somewhat back of center. If it is visible for this placement, it obviously will also be seen when the instrument is leveled.

Duties of a Rodperson

The duties of a rodperson are relatively simple. However, a careless rodperson can nullify the best efforts of an observer by failing to follow a few basic rules. A level rod must be held plumb on the correct monument or turning point to give the correct reading. In Figure 5.2, point A is below the line of sight by vertical distance AB. If the rod is tilted to position AD, an erroneous reading AE is obtained. It can be seen that the smallest reading possible, AB, is the correct one and is secured only when the rod is plumb.

A rod level of the type shown in Figure 5.3 ensures fast and correct rod plumbing. Its L-shape is designed to fit the rear and side faces of a rod, while the bull’s-eye bubble is centered to plumb the rod in both directions. However if a rod level is not available, one of the following procedures can be used to plumb the rod.

Waving the rod is one procedure that can be used to ensure that the rod is plumb when a reading is taken. The process consists of slowly tilting the rod top, first perhaps a foot or two toward the instrument and then just slightly away from it. The observer watches the readings increase and decrease alternately, and then selects the minimum value, the correct one. Beginners tend to swing the rod too fast and through too long an arc. Small errors can be introduced in the process if the bottom of the rod is resting on a flat surface.A rounded-top monument, steel spike, or thin edge makes an excellent benchmark or intermediate point for leveling.

On still days the rod can be plumbed by letting it balance of its own weight while lightly supported by the fingertips. An observer makes certain the rod is plumb in the lateral direction by checking its coincidence with the vertical wire and signals for any adjustment necessary. The rodperson can save time by sighting along the side of the rod to line it up with a telephone pole, tree, or side of a building. Plumbing along the line toward the instrument is more difficult, but holding the rod against the toes, stomach, and nose will bring it close to a plumb position. A plumb bob suspended alongside the rod can also be used, and in this procedure the rod is adjusted in position until its edge is parallel with the string.