The engineers level, often referred to as the "dumpy level," is the instrument most commonly used to attain the level line of sight required for differential leveling (defined later). The dumpy level and the self-leveling level can be mounted for use on a tripod, usually with adjustable legs (figure 5-1).
Mounting is done by engaging threads at the base of the instrument (called the footplate) with the threaded head on the tripod. These levels are the ones most frequently used in ordinary leveling projects. For rough leveling, the hand level is used.
Figure 5-2 shows a dumpy level and its nomenclature. Notice that the telescope is rigidly fixed to the supporting frame.
Figure 5-2.Dumpy level.
Inside the telescope there is a ring, or diaphragm, known as the reticle, which supports the cross hairs. The cross hairs are brought into exact focus by manipulating the knurled eyepiece focusing ring near the eyepiece, or the eyepiece itself on some models. If the cross hairs get out of horizontal adjustment, they can be made horizontal again by slackening the reticle adjusting screws and turning the screws in the appropriate direction. This adjustment should be performed only by trained personnel. The object to which you are sighting, regardless of shape, is called a target. The target is brought into clear focus by manipulating the focusing knob shown on top of the telescope. The telescope can be rotated only horizontally, but, before it can be rotated, the azimuth clamp must be released. After training the telescope as nearly on the target as you can, tighten the azimuth clamp. You then bring the vertical cross hair into exact alignment on the target by rotating the azimuth tangent screw.
The level vial, leveling head, leveling screws, and footplate are all used to adjust the instrument to a perfectly level line of sight once it is mounted on the tripod.
You can save time using the self-leveling, or so-called "automatic," level in leveling operations. The self-leveling level (figure 5-3) has completely eliminated the use of the tubular spirit level, which required excessive time because it had to be reset quite often during operation.
Figure 5-3.Self-leveling level.
The self-leveling level is equipped with a small bulls-eye level and three leveling screws. The leveling screws, which sit on a triangular footplate, are used to center, as much as possible, the bubble of the bulls-eye level. The line of sight automatically becomes horizontal and remains horizontal as long as the bubble remains approximately centered.
The hand level, like all surveying levels, is an instrument that combines a level vial and a sighting device. Figure 5-4 shows the Locke level, a type of hand level. A horizontal line, called an index line, is provided in the sight tube as a reference line. The level vial is mounted atop a slot in the sighting tube in which a reflector is set at a 45° angle. This permits the observer, who is sighting through the tube, to see the object, the position of the level bubble in the vial, and the index line at the same time.
To get the correct sighting through the tube, you should stand straight, using the height of your eye (if known) above the ground to find the target. When your eye height is not known, you can find it by sighting the rod at eye height in front of your body. Since the distances over which you sight a hand level are rather short, no magnification is provided in the tube.
Figure 5-4.Locke level.
SETTING UP A LEVEL
After you select the proper location for the level, your first step is to set up the tripod. This is done by spreading two of the legs a convenient distance apart and then bringing the third leg to a position that will bring the protector cap (which covers the tripod head threads) about level when the tripod stands on all three legs. Then, unscrew the protector cap, which exposes the threaded head, and place it in the carrying case where it will not get lost or dirty. The tripod protective cap should be in place when the tripod is not being used.
Lift the instrument out of the carrying case by the footplate-not by the telescope. Set it squarely and gently on the tripod head threads and engage the head nut threads under the footplate by rotating the footplate clockwise. If the threads will not engage smoothly, they may be cross-threaded or dirty. Do not force them if you encounter resistance; instead, back off, and, after checking to see that they are clean, square up the instrument, and then try again gently. Screw the head nut up firmly, but not too tightly. Screwing it too tightly causes eventual wearing of the threads and makes unthreading difficult. After you have attached the instrument, thrust the leg tips into the ground far enough to ensure that each leg has stable support, taking care to maintain the footplate as near level as possible. With the instrument mounted and the legs securely positioned in the soil, the thumbscrews at the top of each leg should be firmly tightened to prevent any possible movement.
Quite frequently, the Builder must set up the instrument on a hard, smooth surface, such as a concrete pavement. Therefore, steps must be taken to prevent the legs from spreading. Figure 5-5 shows two good ways of doing this. In view A, the tips of the legs are inserted in joints in the pavement. In view B, the tips are held by a wooden floor triangle.
Figure 5-5.Methods of preventing tripod legs from spreading.
LEVELING A LEVEL
To function accurately, the level must provide a line of sight that is perfectly horizontal in any direction the telescope is trained. To ensure this, you must level the instrument as discussed in the next paragraphs.
When the tripod and instrument are first set up, the footplate should be made as nearly level as possible. Next, train the telescope over a pair of diagonally opposite leveling screws, and clamp it in that position. Then, manipulate the leveling thumb-screws, as shown in figure 5-6, to bring the bubble in the level vial exactly into the marked center position.
Figure 5-6.Manipulating leveling thumbscrews.
The thumbscrews are manipulated by simultaneously turning them in opposite directions, which shortens one spider leg (threaded member running through the thumbscrew) while it lengthens the other. It is helpful to remember that the level vial bubble will move in the same direction that your left thumb moves while you rotate the thumbscrews. In other words, when your left thumb pushes the thumbscrew clockwise, the bubble will move towards your left hand; when you turn the left thumbscrew counter-clockwise, the bubble moves toward your right hand.
After leveling the telescope over one pair of screws, train it over the other pair and repeat the process. As a check, set the telescope in all four possible positions and be sure that the bubble centers exactly in each.
Various techniques for using the level will develop with experience; however, in this section we will only discuss the techniques that we believe are essential to the Builder rating.
An engineers level is a precision instrument containing many delicate and fragile parts. It must therefore be handled gently and with the greatest care at all times; it must never be subjected to shock or jar. Movable parts (if not locked or clamped in place) should work easily and smoothly. If a movable part resists normal pressure, there is something wrong. If you force the part to move, you will probably damage the instrument. You will also cause wear or damage if you excessively tighten clamps and screws.
The only proper place to stow the instrument when it is detached from the tripod is in its own carrying box or case. The carrying case is designed to reduce the effect of jarring to a minimum. It is strongly made and well padded to protect the instrument from damage. Before stowing, the azimuth clamp and leveling screws should be slightly tightened to prevent movement of parts inside the box. When it is being transported in a vehicle, the case containing the instrument should be placed as nearly as possible midway between the front and rear wheels. This is the point where jarring of the wheels has the least effect on the chassis.
You should never lift the instrument out of the case by grasping the telescope. Wrenching the telescope in this manner will damage a number of delicate parts. Instead, lift it out by reaching down and grasping the footplate or the level bar.
When the instrument is attached to the tripod and carried from one point to another, the azimuth clamp and level screws should be set up tight enough to prevent part motion during the transport but loose enough to allow a "give" in case of an accidental bump against some object. When you are carrying the instrument over terrain that is free of possible contacts (across an open field, for example), you may carry it over your shoulder like a rifle. When there are obstacles around, you should carry it as shown in figure 5-7. Carried in this manner, the instrument is always visible obstacles may be encountered.
Figure 5-7.Safest carrying position for instrument when
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015