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OP Figure A-12: View of stadia hairs through a level, on a stadia rod 93 feet away. Using stadia, a precision of about 1/500 can be achieved with reasonable care. Errors in stadia work are usually the result of poor rod readings. With this precision, the upper limit for a single stadia- sight distance is about 500 feet. Electronic Distance Meters (EDM's) An electronic distance meter works on the principle of determining the time required for an induced electromagnetic wave to reach a reflec- tor and return to the sender. Automatic equipment converts this time to a distance. Two different types of EOM's are available. The first type is activated by a infrared frequency signal or Tasar beam. The signal is reflected from a bank of retroprisms, and the distance calculated by the time required for the signal to return to the source. Since this type of EDM is "light-frequency" activated, a clear line of sight to the target is required. Foliage, mist, and smoke can interfere with the signal.. Additionally, the target and signal source must be essentially on-line, resulting in a narrow band within which the target must remain in order to obtain a reading. The- second type of EDM broadcasts a microwave to a receiver, which relays the signal back to the source. This type of EOM is unaffected by reduced visihility, but may be jammed by transmission lines or other high energy electromagnetic fields. They are not affected by normal radio transmissions. The feature which makes these meters particularly attractive for hydrographic work is that they can maintain signal contact when the source and receiver units are offset by as much as 450. Thus, it is much easier to prevent loss of signal when moving the boat across the channel. An advantage of the "line of sight" meters is that if contact is broken, they do not need to be recalibrated in order to continue measurements. The price of electronic distance meters is quite high, ranging from about 53000 to $20,000 in 1978 (Appendix 0). However, they are practically indisrensible for workina on large rivers. Even a fl, -7