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Adjusting the EDM on a Topcon PS-101A
I took my instrument to a calibration baseline in West Hartford Connecticut. The results of the effort indicate that the instrument is measuring long, due to scaling at a rate of 11 ppm.
I was under the impression that adjustment of the constant and scaling for the instrument must be performed by the dealer, however an associate of mine indicates that I should be able to do it myself.
Anyone have experience with this?
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27 Replies
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What instrument? There may be differences in what is user-adjustable.
Did you have accurate temperature and pressure settings? If not, then the result is not meaningful. If the settings were right, you could offset by 11 ppm in the future by just fudging the temperature setting.
I’m assuming you did multiple distances on the calibration line so as to know it was really ppm and not a prism constant error.
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Check your settings, offset, temperature, barometric pressure and curvature of earth and such that can give undesired results.
I let the professionals handle the insides of these modern instruments because they have the tools and parts that are needed and their work is certified and guaranteed.
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typically you can adjust the temperature and pressure until you hit your desired ppm result.
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+11 ppm is a typical temperature/pressure setting for this time of year. The PS doesn’t correct for that automatically like other brands do. So I’m wondering if you are entering those values. That is definitely something the user can, and should, do at every setup.
If your instrument needs actual service I’d leave it to the pros.
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Sorry, just seeing the replies now (need to figure out how to get notifications when someone replies).
Bill93, I performed the calibration on a Topcon PS-101A. And yes, we performed multiple measurements on the calibration base line – at every occupy point (we used 4 out of 6), we measured each of the other three points.
I used a Kestrel 3500 pocket weather meter to measure temperature, barometric pressure and relative humidity at the site and entered these on the instrument prior to making the observations. I recorded and entered temperature of 70 deg F (+/- 0.9 deg F), barometric pressure 1006 hPa (+/- 1.0 hPa) and relative humidity at 46 percent (+/- 3 percent). Accuracies are those published by Kestrel. Conditions were fairly consistent during the effort, and all measurements were made within one hour.
I’m attaching the results of the calibration effort to this post (results from Primacode Calibrate).
Prism offset was set to -30mm and we were using -30mm prisms on Sokkia AP41 tribrach adapters (each carefully adjusted the night before).
One thing I need to research is how the earth curvature setting might affect the results. The Carlson RW5 file indicates that earth curvature was off during the observations.
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I sent my instrument to the dealer.
BTW, I just did some calculations on how atmospheric conditions affect EDM measurement. I used the formula from the instrument manual. I find the following:
If the entered value for temperature differed from the true value by 1 deg. F, the approximate scaling effect would be 0.53 ppm. If the entered value for pressure differed from the true value by 1 hPa, the approximate scaling effect would be 0.27 ppm. If the entered value for relative humidity differed from the true value by 1 percent, the approximate scaling effect would be 0.01 ppm. Temperature is most significant, pressure less significant. Humidity is of negligible significance.
The results of my calibration base line effort suggest that scaling was statistically significant at a rate of -11 ppm. If the scaling was due solely to an error in measuring and/or entering the temperature, it would have required an error of approximately 20 deg Fahrenheit. I don’t think I was off by nearly that much. Of course, it could have been a combination of atmospheric errors, but I agree with Bill93’s suggestion that the most significant is temperature.
The formula given by the instrument manufacturer (Topcon) is as follows:
formula for atmospheric correction
The formula indicates that the ppm value will be positive (greater than 0 ppm) when the temperature is higher than 15 deg C (59 deg F), or when the pressure is lower than 1013 hPa or when the relative humidity is greater than 50 percent, or some combination of these factors.
As I understand it, EDM waves moves faster through less dense air. This tends to be air with a higher temperature and/or lower pressure. Instruments are set based upon standard values for temperature, pressure and relative humidity – in the case of my instrument – 15 deg C, 1013 hPa and 50 percent RH, when the PPM correction value is zero. If it is warmer than 59 deg F, and the pressure is less than 1013 hPa, then the EDM wave will move faster relative to the standard values. Distance is determined by the time of flight of the EDM wave from the instrument to the prism and back. So, if the wave is moving faster through the warmer air, it will take less time, which in turn will result in a shorter distance.
At the calibration base line, based on a temp of 70 deg F, a pressure of 1006 hPa, and a relative humidity of 46 percent, I calculate using the formula provided by the manufacturer that the Atmospheric Correction Factor (PPM) value is + 7.88 ppm. The atmospheric conditions indicate less dense air, through which the EDM wave will move faster than in standard air conditions. The correction factor is used by the instrument to increase the raw slope distance, to get the adjusted distance.
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Subscribing to the topic.
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Big Al , I am curious as to how old your instrument is? (10-12 years??)
Also the formula you gave what are “p” “t” and “e” units given in? ( the units above, deg. F, p in hPa and percent for “e”??).
JOHN NOLTON
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Norman O. In your post you say that other brands correct for Temp./pressure automatically?
Topcon S6 did (by reading post here) but what other Total Stations do? I have been wondering this
for some time now.
JOHN NOLTON (old and retired for 20 years and have not kept up with what’s on the market)
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John,
I am the second owner of the instrument. I Purchased it used last fall. I??m not sure exactly how old it is, but I would estimate that it is less than 10 years old.
As far as the formula goes, I believe that the only difference is that temperature is measured in Celsius not Fahrenheit. Hope that helps. It came straight from the instrument manual.
Al
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Al, thanks for the reply. I finally got my computer to download the operation manual for your inst. The manual
was printed in 2012 so your inst. is around 7 years old (at least). I think it could be older.
The last page of your manual gives the address of Topcon Corp. in Japan. You could write them and ask how old
it is. This would be very interesting because in the good old days the aging of the oscillator was about 1ppm/year.
What they will do (repair shop, if they are good) is to hook it to a frequency counter and adjust it back to the factory
frequency. Charlie Glover (NGS retired and deceased) would do this on all of the EDMI at NGS and I watched as
he did a Tellurometer MA100 back in 1980. The ppm formula you gave is on pg. 124.
One thing I would do is IF you might want the best distance your inst. can give you is put it in the meter mode
to measure the distance on the base line. All EDMI measure in meters and convert to feet. So even though you
measure in the foot mode in measured it in meters and converted to feet for you (less accurate)
JOHN NOLTON
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I’m specifically thinking of the Trimbles. I think that the newest Leicas do it, too, but I can’t attest to that.
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John,
Thank you for your post, and for the detailed and interesting information about this subject. It is hard to find much about this on the Internet.
I will try to find out from Topcon what year the instrument was made. I suspect it may be several days before I hear a reply, if not longer.
I have sent the instrument to GeoShack in Ohio. I believe they are one of Topcon’s largest dealers. Hopefully, they have the facility to adjust the frequency to the factory settings.
On previous calibrations, I have set the instrument to read in meters instead of rounding to feet, for precisely the reasons you outlined. The new instrument, a Topcon PS-101A, is more precise, and rounds to the nearest thousandth of a foot, unlike the my previous instrument (a Sokkia SRX3) which rounds to the nearest hundredth of a foot. Rounding to the nearest hundredth is significant relative to the efforts conducted at a calibration base line, where it comes down to thousandths.
Because of this increased precision, and because I usually measure in feet, I decided to avoid setting the distance units to meters for this exercise.
I have a question John – perhaps you know. Is the frequency adjustment performed digitally through software, or mechanically by tuning a screw, etc?
Al
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While I’m questioning just about everything about how I performed the calibration, and what might be the cause of the scaling, I am confident that I measured the atmospheric conditions at the time of the calibration, and that i entered the atmospheric conditions into the instrument through the survce interface. I also checked to see that the atmospheric values made it to the instrument, and found they did.
BTW, I like the Kestrel pocket meter. It is compact, inexpensive, can be calibrated and certified and easily fits with in the case of the total station.
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John,
From what you’ve seen or learned, is there a tendency for the frequency of an oscillator to drift upward over time? If I understand it correctly, if the frequency is higher than factory, then there will be more wavelengths for a given amount of time, thus more distance. Instrument will measure long.
Al
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Al, I am having trouble posting but I hope this goes through on another computer.
Look at Dr. Rueger book “Electronic Distance Measurement, an introduction, 4th edition on page 181 (top).
It will tell you about how the long-term stability ( ageing) will affect the EDMI (drops with time) and more.
JOHN NOLTON
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John,
Thanks for referencing Dr. Rueger’s book. I have just now ordered a used copy of the third edition. Can’t justify spending over $100 on the fourth edition.
BTW, called my dealer, and spoke with the service manager. He said that the frequency of the 690nm laser diode is “fixed” and cannot be adjusted. But, he said that he can nevertheless adjust for scaling. I presume it is some sort of digital adjustment. He said the adjustment cannot ordinarily be accessed by the end user. Instrument must be entered into some sort of factory adjustment mode, presumably only available to Topcon dealers. I’ll see what comes of it.
Al
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Alex:
Years ago with the old manual EDM/Theodolite combinations we just tweaked the prism constant to compensate for minor differences in readings.
You have sent your instrument out for professional repair so hopefully you are all set. I probably would have returned to the baseline with a different prism and checked again just be sure I wasn’t having a bad day with setups. etc.
Hope all is well in South County.
Tom
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Thanks, Tom. My dealer said he’ll be able to tell me whether he agrees with our baseline effort. I’ll post here again once I hear back.
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0.003 feet difference from a published distance is an acceptable degree of accuracy for most measurement purposes and is a good result on any baseline.
How does it check between points you occupied, say a year or more ago.
That is how I check my instruments, by daily occupying former points and find my today’s data and compare to previous occupations before I continue a add to an existing network of control points.
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