SignalsSignals are those survey targets that either areilluminated by natural sunlight or are electrically lightedby use of wet or dry cell batteries. The observations forall first- and second-order triangulation and first-ordertraverse are usually done at night using signal lights,because of more stable atmospheric conditions, whichallow for better pointings. Observations maybe madeduring daylight hours using lights, but for high-accuracysurveys, this is done only under extreme conditions.Some examples of signals are signal lights (fig.15-21), a target set (fig. 15-22), and a heliotrope (fig.15-23). The target set is a precise lighting device that isgenerally used for short traverse lines. The heliotrope isa device that reflects the rays of the sun through a pairof mirrors set over a point and toward an observer onanother station. When standard signals are not available,expedient lights can be used. Examples of expedientlights are the headlights of a vehicle, a masked lantern,or a boxed light bulb.Supports (Towers)Towers must be built on some stations to raise thelines of sight to clear obstructions or to lengthen the linesFigure 15-24.-Pole tower.Figure 15-25.—Aluminum or steel tower.of sight to increase distances between stations of areasurveys. A tower consists of an instrument stand (innerstructure) and a platform to support the observer (outerstructure). Towers fall roughly into three classes:prefabricated aluminum or steel, wooden, and expedienttowers. The towers are usually constructed by a separatecrew, whose size depends upon the type of tower beingbuilt. The expedient tower is usually a tower or highstructure that is already in the area.Two examples of towers are shown in figures 15-24and 15-25.TRIANGULATION PROCEDURESA triangulation survey usually involves thefollowing steps:1. Reconnaissance—meaning the selection of themost feasible points for stations2. Signal erection on these points15-31
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