The light wave measuring device (fig. 12-25) uses electro-optical instruments to measure distances accurately. The device consists basically of two units: the measuring unit (transmitter/ receiver) and the reflector unit (fig. 12-26). The distance is measured by precise electronic timing of a modulated LIGHT WAVE after it travels to, and when it returns from, a reflector at the other end of a course (fig. 12-27). When the instrument receives the reflected light flash, it registers readings that can be converted into the linear distance between the instrument and the reflector (with corrections made for atmospheric conditions).

Like their microwave counterparts, the light wave distance-measuring devices are capable of first order base lines in triangulation and all orders of traverse distance measurements. Most of these instruments have a rated range of 200 to 50,000 meters.

These instruments, as all delicate scientific equipment, are to be treated with proper care and operator maintenance so that they may continue to be available for use. Refer to the instrument manufacturers manual for instructions on basic operation, care, adjustments, calibrations, and other details of the system.

A surveying field party is frequently working its way through rugged terrain a long distance away from any professional medical assistance. Working through brush, felling trees, scaling bluffs, and crossing streams are all hazardous.

Also, the use of such sharp-edged tools as machetes, brush hooks, axes, and hatchets is equally hazardous. Besides those dangers that are inherent in the work itself, a party may be exposed to a variety of natural dangers, such as those created by weather conditions, by reptiles, by insects, and by poisonous plants. Occasionally, in some areas, there may be dangerous wild animals or even dangerous domestic animals, such as vicious dogs or angry bulls. When a party is working along a thoroughfare on which vehicular

traffic is proceeding as usual, there is the ever-present idanger of being hit by a vehicle. In the midst of such a variety of constant dangers, the only way to prevent injury is by the exercise of continual care and vigilance. Every person in a party must be aware of all existing hazards, be able to recognize a hazardous situation, and be trained to take the correct preventive measures.

Indeed, it is common practice for surveying field crews to prepare a CHECKLIST of essential items, personal protective equipment, communication gear, and other miscellaneous items relative to their line of work.