Surveying accessories include the equipment, tools, and other devices used in surveying that are not considered to be an integral part of the surveying instrument itself. They come as separate items; thus, they are ordered separately through the Navy supply system.

When you run a traverse, for example, your primary instruments may be the transit and the steel tape. The accessories you need to do the actual measurement will be the following: a tripod to support the transit; a range pole to sight on in line; a plumb bob to center the instrument on the point; perhaps tape supports if the survey is of high precision; and so forth. It is important that you become familiar with the proper care of this equipment and use it properly.

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.

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 engineers 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 60⁰ with 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

pivot point. When the two legs look about as far away from the mark as the third one and all three are about equally spaced, you lower the two legs and press them into the ground. Make any slight adjustment to level the head further by moving the third leg a few inches in or out before pressing it into the ground.

On smooth or slippery paved rock surfaces, you should tighten the tripod legs hinges while setting up to prevent the legs from spreading and causing the tripod to fall. You should make use of holes or cracks in the ground to brace the tripod. In some cases, as a safety factor, you should tie the three legs together or brace them with rock or bushes after they are set to keep them from spreading. If setups are to be made on a slippery finished floor, rubber shoes may be fitted to the metal shoes, or an equilateral triangle leg retainer may be used to prevent the legs from sliding.

When you are setting up on steeply sloping ground, place the third leg uphill and at a greater distance from the mark. Set the other two legs as before, but before releasing them, check the stability of the setup to see that the weight of the instrument and tripod head will not overbalance and cause the tripod to slip or fall.

Proper care must be observed in handling the tripod. When the legs are set in the ground, care must be taken to apply pressure longitudinally. Pressure across the leg can crack the wooden pieces. The hinge joint should be adjusted and not overtightened to the degree that it would cause strain on the joint or strip or lock the metal threads. The machined tripod head is to be kept covered with the head cover or protective cap when not in use, and the head should not be scratched or burred by mishandling. When the tripod is in use, the protective cap is to be placed in the instrument box to prevent it from being misplaced or damaged. Any damage to the protective cap can be transferred to the tripod head. Mud, clay, or sand adhering to the tripod has to be removed, and the tripod is to be wiped with a damp cloth and dried. The metal parts should be coated with a light film of oil or wiped with an oily cloth. Foreign matter can get into hinged joints or on the machined surfaces and cause wear. Stability is the tripods greatest asset. Instability, wear, or damaged bearing surfaces on the tripod can evolve into unexplainable errors in the final survey results.