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
Al,?ÿI am having trouble posting but I hope this goes through on another computer.
Look at Dr. Rueger book?ÿ "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.
?ÿ
JOHN NOLTON
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
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
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.
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.
If you are referring to the calibration report that I posted, the "residual" ranges from -.006 feet to +.005 feet, but please note that is only AFTER the least squares adjustment has been performed, i.e. scaling has been applied.?ÿ The actual unadjusted residual is calculated from that report by subtracting the "published" value from the "measured" value.?ÿ For example, the published measurement from the 0 meter station to the 770 meter station is 2525.773 feet, and the measured value was 2525.792.?ÿ The difference is +0.019 feet.?ÿ Not much, and negligible for most purposes, but enough to make me want to get the instrument adjusted.
This is the first time that I have performed a calibration effort with this recently acquired instrument.
That's an odd statement from the service manager, about not being able to adjust the frequency of the 690nm laser diode. The frequency of the (infrared) light emitted by the laser is irrelevant for the accuracy of the EDM readings. For prism-mode measurement, what matters for the scale error are the frequencies of the signals that are modulated onto the laser light. The EDM will use multiple frequencies to perform one measurement, and these frequencies are derived from an oscillator with a frequency on the order of 10 megahertz. That's what could be measured and adjusted to correct the scale error. Whether any particular total station provides the means to make that measurement (e.g. provides a test terminal to which a frequency counter could be attached) or the means to adjust the oscillator (e.g. a trimmer capacitor or some kind of digital trimming), I can't say. I know that at least one of my older total stations does have such a test terminal. It's also possible that modern total stations could take a different approach, of letting the oscillator frequency be whatever it is, and then use the results of multiple measurements over known baselines so that the total station's firmware could apply computed corrections.
A difference of 0.019 feet is not an unusual difference between points that were measured month and/or years apart.
I have been checking into control in some areas from 200 feet apart to over 5k feet apart with EDMs since the early 1980s and it is always a ± situation of that amount and in one area between a county road and a lake meander boundary the points at the bottom of the hill are typically sliding into the lake where there are sandbars being created at a very fast rate.
A recent project, I revisited a boundary that in 1894 was 390.01ft, in 2004 was 390.11ft, in 2008 was 390.14ft and last week 2019 was 390.23ft, all between the same original monuments.
Then there is one set of monuments that are over 7k feet apart that were chained between in 1975 that were checked in 1992 with EDM with a difference of -0.02ft and again in 2001 with GPS with a difference of +0.02ft.
I understand that baselines are constructed in a manner and from materials that are sufficient in design and strength to reduce the possibility of any such drift.
I do however do expect that change is certainly possible.
I also have not owned a TS or EDM that the display would read more than two decimal places while the raw data would report to whatever the decimal setting of the unit was set for.
Al , ?ÿ I would like you to ask the dealer 4 questions and put it on paper. Also get it to him before you get the instrument back.
1. carrier wave length
2. main modulation frequency
3. unit length of the EDMI
4. reference refractive index ( we know this by you giving the ppm formula to us. It should be 1.000282324 but don't tell them you know it).
Don't worry if you do not understand all the above. You will when you get Dr. Rueger's book.
?ÿ
Thanks,?ÿ JOHN NOLTON
?ÿ
Thank you for your thoughts on this. I agree that change is possible. That said, this baseline is on very flat ground, and as of yet, I don't think I have enough evidence to warrant a conclusion that the scaling is due movement of the marks.
I actually performed two calibration efforts on the same day at this base station, one with the instrument that is the subject of this post, and another with my "old" instrument, a Sokkia SRX3 total station. The effort indicated that the latter instrument was also measuring long due to scaling at a rate of 6 ppm, but that result was the same as past efforts and of a magnitude only about half as much as the scaling on the Topcon PS-101A. I'm continuing to work with the Sokkia instrument while I get things sorted out with the Topcon.
For what its worth, I used SurvCE to control both instruments. I'm not aware of a setting within SurvCE that would influence the number of decimal places recorded in the raw data file. With the Sokkia, when units are set to u.s. feet, it displays to the nearest hundredth of a foot, and the raw data is to the nearest hundredth of a foot. With the Topcon instrument, it displays to the nearest thousandth of a foot, and the raw data is to the nearest thousandth.
In all fairness, the service manager was answering a (rather dumb) question I had asked him - whether his process was to adjust the frequency of the EDM wavelength. Now I see from your post that there are multiple frequencies and components involved. And it seems like what he told me is more or less correct with respect to the laser diode. I'm looking forward to getting a copy of Rueger's book on the subject and learning more.
By the way, I think you've used the West Hartford baseline, if I'm not mistaken. Do you have any knowledge of the marks having moved? Do you know others that regularly use this base line?
No, I've not used any of the baselines. The nearest one to me is in Marlborough, MA, in the median of an Interstate highway. If it's where I think it is, I'd be surprised if it's still intact. Even if it is intact, some significant clearing would be needed. I'd be interested to hear from anyone if I'm mistaken.
