Home » GIAA Mailbag: On EDM Calibration and Protecting a Total Station
Answers to questions on EDM calibration and protecting a total station from sun damage.
A: In short, your dealer is right. EDMs have errors that are scalar (proportional to the distance being measured), and others where the magnitude is not proportional to the measurement. Both kinds can be systematic (i.e. they always exist in the measurement and can be reasonably predicted), and random (i.e. the magnitude and sign vary with each measurement). Calibrating your EDM against a single reference distance only makes it very accurate at that distance. Worse, the random error components may have conspired with the systematic errors on the particular calibration measurement you made to completely eliminate or magnify the errors. Your dealer calibrates the instrument to perform well over a significant part of the measuring range of the instrument. An even better way is to calibrate using an EDM baseline. These baselines are set up across the country by the NGS or other competent agencies expressly for the purpose of collecting the vital statistics to enable determination of the EDM’s accuracy. A further problem with calibrating your EDM by using a 100-foot tape is that sometimes a fiberglass tape is used. These tapes have much lower accuracy than steel tapes. Regardless of the tape material, GIAA members often find that the methods of measuring a 100-foot distance so that it is horizontal and properly measured utilizing all the taping corrections are rarely applied. Unless the tape itself has been calibrated, even all of these corrections may be for naught. To summarize, EDMs must be calibrated over a series of distances representative of the range of the instrument. The reference distances themselves must be established using rigorous procedures traceable to NIST (National Institute for Standards and Technology).
Q: I’ve been told that pointing a total station telescope at the sun without a protective filter exposes the electronics to damage. I recently paid a significant sum to have my instrument repaired for this problem even though I’ve never done a solar observation with it. How could this have happened?