I have been using EDMs for over 40 years and I've never seen this happen before.
Was recently doing a boundary & topo and I'm now doing the mapping. Ran across one point that's several dozen feet from where I know it should be.
Start comparing to other shots around this one and find that it was about twice as far from the instrument as it should have been. So I decide to calc the location using the horizontal and vertical angles recorded but using a slope distance half of what's in the DC file. The resulting point ends up right where it makes sense to be horizontally and vertically (had a Z angle of 85-36-35, so if elevation didn't make sense, it would have been readily apparent).
With the very early EDMs, you had to dial in different wavelengths or you would get very wrong distances. Those were a few years before my time, so only saw a demonstration. With the earliest ones I used, you needed to be careful about other reflective surfaces near the line of sight or you could get a distance to the chrome bumper or window of a car parked behind the target. The manufacturers seemed to have fixed that issue in the mid 80s.
Have you ever had an EDM give an error of near exactly 2x? Before today, I wouldn't have thought it possible. The only explanation I can imagine is that it read a bounce through the prism back to the instrument lens (that would be 2x less the offset from the IR emitter to the lens).
Using a TS16.
I have never seen a 2X error but I have seen errors in meters. Either in the single meter, 10s of meters or 100s of meters as I believe the distance is resolved using different wavelengths for each of these distances. I would have guessed your error to b a multiple of 1 of these units.
Can't say I've seen that before. Back in the 90's I did have a crew, using a T2002, bring back some topo data where a large number of shots had almost identical distances. It was an obvious error. Turns out that they had been shooting through a road barricade. While the crosshairs were centered on the target, the beam from the top mounted EDM was picking up the reflective tape of the barricade.
Yes/no. On three occasions over a few years I had what I determined to be a 10 meter extra length happen. The first time was obvious as I knew I was shooting approximately 30 feet and the number was a bit more than 60 feet, resembling your situation. Another case said the prism was 90 some feet away from me and I knew it was more like 55 or 60 feet. Don't recall exact distances. Miscounting the number of 10-meter increments made some sense.
Back in the 90s, using 3 different Leica SET2 instruments, I would occasionally get a bad distance. It was too rare to figure out how or why. It happened just as often in the woods as along a road. I never thought of checking for multiples of meters as a possibility.
Back around the turn of the century I worked for an outfit that ran 6 crews, all outfitted with identical Trimble 5603s. One crew, and one crew only, would occasionally (1 shot in a thousand±) get topo shots that were maybe 10x longer than they should have been. Always just one isolated shot. Totally random. No explanation for this was ever found.
In the early 90's i was using a Wild TS with a REC module, I can't remember the model no. where short sights, maybe less than 8 feet would consistently record as double the slope distance. We developed work-arounds but never really figured it out.
Have you ever had an EDM give an error of near exactly 2x? Before today, I wouldn't have thought it possible. The only explanation I can imagine is that it read a bounce through the prism back to the instrument lens (that would be 2x less the offset from the IR emitter to the lens).
An error of exactly 2X (or 0.5X) is pointing to cycle ambiguity error
Your EDM sends out a modulated electromagnetic wave, reflects it off a prism, and compares:
Outgoing modulation wave
Returned modulation wave
The returned signal comes back shifted in phase because it traveled a distance. The equation for the distance is
D= (Measured Phase shift/2 Pi) x (Modulation Wavelength/2)
The phase only tells you if you are within one modulation cycle. Like a clock: If a hand points to 3 o’clock, you know where it is in the cycle… But not whether it’s 3 AM, 3 PM, or 3 the next day.
Same issue with EDM. If modulation wavelength corresponded to, say, 100' ambiguity interval a target at: 50', 150', 250' could produce identical phase observations. This is the cycle ambiguity and EDMs resolve this by using multiple modulation frequencies. What happens when ambiguity is resolved incorrectly
If the instrument adds one extra ambiguity interval: True distance 100' - instrument computes 200'. Exactly double. That’s why doubling can be a signature of ambiguity resolution failure. Conceptually:
Measured phase says: “you’re 0' into this cycle”. Instrument wrongly assumes: cycle #2 instead of cycle #1. Result: distance doubles.
How does incorrect phase resolution can happen. The most likely cause for it happening rarely and randomity is weak signal return. Dirty prism, fog, rain, heat shimmer, etc. can cause the EDM to misidentify the phase. Also, noisy returns: steel girder reflections, glass reflection, reflection off rails.
But this gives you random, not exactly doubled distance error. Integer ambiguity error does. Like using chaining pins and miscounting them.
You can see these in monitoring using AMTS being caused by loss of lock and incorrect reacquisition. The AMTS tracks the prism continuously. If it briefly loses lock due to:
Passing obstruction (equipment, personnel, birds, dust)
Vibration of the prism target
Rapid target movement
Heat shimmer
Low signal return
…it may reacquire a weak or false return and the EDM can resolve the wrong distance. This can cause sudden doubled distance outlier, one-epoch spikes and occasional “teleporting” target positions, all common patterns in large monitoring data sets.
So, my guess is: 1) Reacquisition cycle-slip, 2) False return / multipath, or 3) Tracking + vibration induced ambiguity slip if it doesn't happen again and internal instrument electronics failure (oscillator drift, frequency synchronization issues) if it does.
While the crosshairs were centered on the target, the beam from the top mounted EDM was picking up the reflective tape of the barricade.
I was and I'm still concerned about side view vehicle mirrors.
An error of exactly 2X (or 0.5X) is pointing to cycle ambiguity error
...
Jim, thanks for that. I thought about a cycle error. I've encountered it once or twice, but not in the last 40 years. With a cycle error, I would expect the error to show up in multiples of the EDM's measuring wavelength. So, hypothetically, if the wavelength is 10.000 m, I would expect the error to be 39.37' or multiples thereof.
So if the point is 52.05' from the instrument, the phase shift is 12.68' (52.05' - 39.37') and a reported distance of 91.42' would indicate a cycle error of one cycle. How does the phase shift also get multiplied with this type of error?
I'm digging back into my memory 40+ years so I could be, and apparently am forgetting something rather basic.
However, the conditions at the time was off & on misty rain and my I-man was complaining about the lenses fogging up, so that fits right in with your explanation.
Had something similar happen on an older Leica once and it ended up being moisture inside the prism connector causing random distance spikes. Took forever to track down because everything else checked out fine. Also worth checking if the error only shows up at certain distances or angles - sometimes that helps narrow it down fast.
I had a similar problem when I was working in Germany. I showed up on site and realised that I had forgotten my circular prism. I only had my 360 prism with me. So, I traversed around the site using the resection method. On one particular setup I read back to the 3 previous control points I had placed from the previous station. The resection result was way off. But upon checking the residuals, it was clear which point was causing problems. I deleted it from the mix and re-iterated. The resulting scale factor was so close to perfect that I didn't bother searching for a third point. When I got home I checked my data in CAD. As it turns out, the bad control point had been placed very close to the instrument. The instrument's EDM had read to a point approx 2m beyond the prism. The 360 prism was Chinese. I tested the process with my genuine Leica prism and have not been able to reproduce the issue since. That was in 2012/2013.