Tool tips for damaged prisms and calibrating a 100-ft steel tape.

Q: I have several prisms that are damaged with miscellaneous cracks, chips and breaks. I know that such damage reduces EDM range, but are there any other negative impacts? I haven't been able to detect any from my survey results.

A: In general it is best to discard damaged prisms. You could encounter many types of problems in your EDM measurements. If the damage is a crack, it is possible that the reflection of light occurs at one or more places within the prism where the cracked surfaces come together as well as at the back surfaces of the prism; this can lengthen or shorten the observed distance depending on the path the light takes inside the prism. Remember, a prism is not a mirror, so damage to the front surface, back surfaces or in-between can all impact the measurement. And yes, you are correct, it is likely that the amount of light reflected is reduced, thus reducing EDM range. A chip such as a gouge in the front surface of the prism glass will definitely reduce the amount of light returned to the total station and thus reduce the range. The light traveling into the prism through this gouge may or may not be returned to the total station. If it is returned to the total station, it is likely that it will have the effect of "signal noise" perceived by the total station and will likely reduce the accuracy the EDM is capable of producing . It may well be that the effective prism constant is changed by such damage as well. You may be able to get away with using prisms with damaged glass in shorter-range measurements for such applications as topographic surveys, but you are better off replacing them with prisms free of blemishes to ensure accuracy.

Q: What is the best way to calibrate a 100-ft steel tape? I have a contract with a government agency that requires a calibrated steel tape.

A: Government agencies and certain testing and standards laboratories will calibrate a tape for you, but you can also do it yourself. To do it yourself, you need a tape baseline that is about an order of 10 more accurate than you need. If you need to measure to one-hundredth of a foot, this means a baseline calibrated to about one-thousandth of a foot. Many government agencies, especially state agencies concerned with surveying, may already have an established taping baseline. Universities and colleges are another source. If an EDM baseline exists in your vicinity, check it out. It may be that there is a taping baseline established at right angles to the "zero" monument. If none of these options are available, you will have to get your hands on a calibrated tape. An "Invar" tape that has a low coefficient of thermal expansion is preferred for this work, though a standard calibrated steel tape will work, too. The idea is that there must be traceability from an accepted standard to your tape. You can set up a baseline by measuring 100.00 ft with the calibration tape and setting brass disks or plugs in concrete or some other durable material. Take care to ensure that movement of the end points does not occur. Now use the standard taping corrections, a tension handle and thermometer. You should also use a small wooden or metal scale (frequently available from tape manufacturers) that serves as a vernier to accurately measure to 0.001 ft. If you don't have such a scale, you can make your own after studying the theory of a vernier. To establish the baseline, a single measurement will not do. Take several measurements-perhaps 10 or more, and then take another set on another day under different temperature and tension conditions. All these measurements should compare well against each other. In the end, you will have a baseline with a known distance, and it will be rarely 100.00 ft in length. Finally, you can calibrate your tape. As with the process of determining the baseline's length, you should also use a tension handle, thermometer and vernier scale. It is then a simple process to transfer the true length of the baseline to the tape to determine the tape's true length. Remember that when a tape is calibrated, the calibration conditions (temperature, tension, and whether end supported or fully supported, etc.) must also be noted.