Answers to questions on EDM calibration and protecting a total station from sun damage.

Typical total station telescope EDM components. Graphic supplied by Kerry Kemper of PENTAX Corporation.
Q: I calibrate my EDM by making a check measurement against a 100-foot tape. I adjust any discrepancies by changing the prism constant in 1 mm steps. This method seems to work and saves me a lot of time and effort over taking it to a dealer for adjustment. My dealer tells me this is not the correct method and can be responsible for introducing more error into EDM measurements than I am attempting to correct. Who is right?

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?

A: You do not have to do a solar observation to expose the electronic, optical and even mechanical components of a total station telescope to damaging rays from the sun. Service departments at GIAA member companies have heard stories about telescopes that were left pointed at the sky such that the sun eventually moved to be aligned with the optical axis of the telescope, just by coincidence. Properly done, solar observations with a total station are perfectly safe, for your eyes and the total station. But the observation must be done with a protective filter placed over the objective lens so that the harmful rays do not enter the telescope. Eyepiece filters are available for theodolites, which protect the human observer. But these are not intended for total stations, which have more sensitive components in the telescope. Since the EDM uses the telescope to project and receive infrared light, many components may be found along the light path of the telescope, which may be burned or damaged by the concentrated rays of the sun. The telescope acts in many ways like a magnifying glass to concentrate the sun’s energy on many of the EDM’s critical components. Filters may be melted or vaporized; lens coatings and cements may be melted. While this damage can cause failure or malfunction of critical components, a further aggravating problem is that the vaporized materials may contaminate other components of the EDM or the visual optics.

Have a question for the GIAA Mailbag? Please E-mail your questions to Diana Brown at brownd@bnp.com. Your questions, and their responses from qualified professionals, will help others. You may also ask your questions or continue the discussion at www.rpls.com.