Caring for and Maintaining a Survey Tape
If a steel or metallic tape gets a kink in it, it is then subjected to strain. The tape at best will be distorted at the point where the kink lies. At worst, if the strain is strong enough, the tape will break at the point where the kink lies. Kinks, therefore, are to be avoided at all costs; it is especially important to avoid putting a strain on a tape with a kink in it.
Under favorable circumstances, when a tape is shifted ahead, the head chainman may simply drag it over the ground. It is not a good idea for the rear chainman to assist by dragging that end because this develops a curve in the tape. This curve may snag on an obstruction and also may be the cause of a kink. When a tape is being dragged, the rear chainman should simply allow the end to trail along. The cardinal rule is "keep, the tape straight."
When taping in traffic, you plan your moves in advance and make the measurement as fast as possible. If possible, do not let vehicles run over the tape; however, if this is absolutely unavoidable, be sure the tape is laid flat and taut on the road. NEVER let a vehicle run over a tape laid on a soft or rugged ground surface.
Tapes are made as corrosion-resistant as possible, but no steel tape is entirely immune to corrosion, especially when used around salty water. Therefore, a tape should always be wiped dry before it is put away, and it should be oiled periodically with a light, rust-resistant oil. If a tape does rust, rubbing it with light steel wool dipped in a rust-removing compound is the best and safest way to remove the rust. Tapes, especially those in reels, though not used during the week, should be removed from the reel and inspected each week for signs of corrosion. A damp climate in your area of operations could easily start corrosion in tapes.
Splicing a Tape
In spite of being carefully handled, tapes sometimes break. A broken tape is rejoined by splicing. A relatively light tape can be repaired with a punch-and-rivet tape splicer and repair stock (fig. 12-22). A repair stock consists of a
Figure 12-22.-A punch-and-rivet tape splicer with repair stock.
length of tape of the same thickness and width as that of the broken tape. When a tape is repaired, it is best to use a good section of the tape for calibration (matching a whole-foot mark). Place the section used for calibrating beside the broken section to make sure that you will maintain the original length of the tape after rejoining it.
In splicing a broken tape, first align and rivet the repair stock at one end of the break. Next, place the repair stock on the face of the other section of the tape by using the calibrating section as a measure for the break splice. Insert one rivet at a time and arrange rivets in a triangular pattern.
Do not place rivets closer together than one-fourth in. from center to center. Now use a three-edge file. File partially through the surplus stock diagonally across the tape. The segment of the surplus will readily break off, leaving a clean splice.
Figure 12-23.—A microwave distance-measuring device (Model 99).
Heavy steel tapes are repaired in a similar manner, using the tape repair kit shown in chapter 11, figure 11-55.
MEASURING BY THE ELECTRONIC DISTANCE-MEASURING SYSTEM
The electronic distance-measuring system is now incorporated in various present-day surveying practices, including traverse and triangulation network. In traverse measurements, accurate distances are directly measured in a straight line and with minimum instrument setups. In triangulation, the system is used to conduct base line measurements that are precise enough to maintain the accuracy of the survey.
In the electronic distance-measuring system, the length of a linear interval is determined by the use of equipment that (1) sends out an electronic impulse of some sort, such as a radar microwave or a modulated light wave, and (2) measures the time required for the impulse to travel the length of the interval.
The velocity or rate of travel of the impulse is known. Therefore, once the time is also known, the length of the linear interval can be determined by applying the well-known equation "distance = rate x time."
Two types of electronic distance-measuring devices (also called EDMs) commonly used today are the MICROWAVE DEVICES and the LIGHT WAVE DEVICES.