Tripod

Tripod The tripod is the base or foundation that supports the survey instrument and keeps it stable during observations. A tripod consists of a head to which the instrument is attached, three wooden or metal legs that are hinged at the head, and pointed metal shoes on each leg to be pressed or anchored into the ground to achieve a firm setup. The leg hinge is adjusted so that the leg will just begin to fall slowly when it is raised to an angle of about 45⁰. The tripod head may have screw threads on which the instrument is mounted directly, a screw projecting upward through the plate, or a hole or slot through which a special bolt is inserted to attach to the instrument. Two types of tripods are furnished to surveyors: the fixed-leg tripod and the extension- leg tripod. The fixed-leg type is also called a STILT-LEG or RIGID tripod, and the extension- leg tripod is also called a JACK-LEG tripod. Both types are shown in figure 11-37. Each fixed leg may consist of two lengths of wood as a unit or a single length of wood split at the top, attached to a hinged tripod head fitting and to a metal shoe. At points along the length, perpendicular brace pieces are sometimes added to give greater stability. The extension tripod leg is made of two sections that slide longitudinally. On rough ground, the legs are adjusted to different lengths to establish a horizontal tripod head or to set the instrument at the most comfortable working height for the observer. A leg may be shortened and set as shown in the extreme right view of Figure 11-37.-Types of tripods. The fixed legs must be swung in or out in varying amounts to level the head. Instrument height is not easily controlled, and the observer must learn the correct spread of the legs to get the desired height. WIDE-FRAME tripods, like those shown in figure 11-38, have greater torsional stability and tend to vibrate less in the wind. You should grip the surveying instrument firmly to avoid dropping it while you are mounting it on the tripod. Hold the transit by the right standard (opposite the vertical circle) while you are attaching it. The engineer’s level should be held at the center of the telescope, while theodolites and precise levels should be gripped near the base of the instrument. The instruments should be screwed down to a firm bearing but not so tightly that they will bind or the screw threads will strip. In setting up the tripod, you should be sure to place the legs so that you achieve a stable setup. On level terrain, you can achieve this by having each leg form an angle of about 60⁰with the ground surface. Loosen the restraining strap from around the three legs, and secure it around one leg. An effective way to set the tripod down is to grip it with two of the legs close to the body while you stand over the point where the setup is required. By using one hand, you push the third leg out away from the body until it is about 50° to 60° with a horizontal. Lower the tripod until the third leg is on the ground. Place one hand on each of the first two legs, and spread them while taking a short backward step, using the third leg as a Figure 11-38.-Wide-frame tripods. 11-38 figure 11-37.