A primary triangulation station is both a sighted secondary triangulation station is one that is sighted from primary stations but is not itself used as an instrument station. Only the primary stations are used to extend the system of figures.

Each triangulation station must be marked in a way that will make it visible from other stations from which it is sighted. A mark of this kind is called a triangulation signal. For a secondary station, the signal may be relatively simple, such as a pole set in the ground or in a pile of rocks, or a pole set on the ground and held erect by guys. An object already in place, such as a flag pole, a church spire, or a telegraph pole, will serve the purpose. When the instrument itself must be elevated for visibility, a tower is used.

A target is generally considered to be a nonilluminating signal. Target requirements can be met by three general typestripods, bipeds, and polesall of which may incorporate variations. The targets are constructed of wood or metal framework with cloth covers.

SIZE OF TARGET. For a target to be easily visible against both light and dark backgrounds, it should be constructed in alternating belts of red and white or red and yellow. For ready bisection, it should be as narrow as possible without sacrificing distinctness. A target that subtends an angle of 4 to 6 seconds of arc will fulfill this purpose. Since 1 second of arc equals 0.5 centimeters at a 1-kilometer distance, an angle of 6 seconds requires a target 3 centimeters wide at 1 kilometer or 30 centimeters at 10 kilometers. Under adverse lighting conditions, the target width will have to be increased. Flags of an appropriate size may be added to aid in finding the target. All cloth used on targets should be slashed after construction to minimize wind resistance.

TRIPOD TARGET. The tripod target is the most satisfactory from the standpoint of stability, simplicity of construction, durability, and accuracy. It ranges from a simple hood of cloth, cut and sewn into a pyramid shape and slipped over the instrument tripod, to the permanent tripod with the legs embedded in concrete, sides braced, a vertical pole emplaced, and the upper part boarded up and painted. Temporary tripod targets may be constructed of 2-inch by 2-inch lumber, pipes, poles, or bamboo joined at one end by wire or bolts threaded through drilled holes. The tripod must be

Figure 15-18.Tripod targets.

BIPOD TARGET. Bipod targets are more simply constructed than tripods but are less stable and must be strongly guyed. Figure 15-19 shows a standard surveying biped target. It is carried disassembled in a canvas case about 53 inches long. It can be assembled, erected, and plumbed by two men in 15 minutes. If this target must be left standing in the weather for any extended period, the rope guys should be replaced with wire and two more wire guys added to each end of the crossbar. In soft ground the pointed legs will sink unevenly because of wind action and rain; therefore, they should be set in holes bored in the end of wooden stakes driven flush or in a short piece of 2-inch by 4-inch lumber laid flat in a shallow hole.

POLE TARGETS. Pole targets (fig. 15-20) are seldom used because the station cannot be occupied while the target is in place. In certain cases, as when an unoccupied station must be sighted and cutting of lines of sight is difficult or impossible, a pole target that can be seen above the trees maybe erected The staff may be constructed of 2-inch by 2-inch lumber or cut poles, varying from about 2 inches to 6 inches in diameter. The method of joining sections of 2-inch by 2-inch lumber and the construction of a panel target are shown in figure 15-20. The targets must be plumbed by manipulation of the guy wires. Special care must be taken when warped or crooked boards are used to construct pole targets, and they must be checked for eccentricity.