Signals are those survey targets that either are illuminated by natural sunlight or are electrically lighted by use of wet or dry cell batteries. The observations for all first- and second-order triangulation and first-order traverse are usually done at night using signal lights, because of more stable atmospheric conditions, which allow for better pointings. Observations maybe made during daylight hours using lights, but for high-accuracy surveys, 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 is generally used for short traverse lines. The heliotrope is a device that reflects the rays of the sun through a pair of mirrors set over a point and toward an observer on another station. When standard signals are not available, expedient lights can be used. Examples of expedient lights are the headlights of a vehicle, a masked lantern, or a boxed light bulb.

Towers must be built on some stations to raise the lines of sight to clear obstructions or to lengthen the lines

prefabricated aluminum or steel, wooden, and expedient towers. The towers are usually constructed by a separate crew, whose size depends upon the type of tower being built. The expedient tower is usually a tower or high structure that is already in the area.

Two examples of towers are shown in figures 15-24 and 15-25.