Using a total station on a simple lot/block survey. We had reconned with a tape to locate record monuments. While set directly on one of them I shot to another that had to be close to 150 feet away. The screen showed 182.67 feet.
What had happened was that the shot was passing through a small maple tree with small limbs and large leaves. The focus had already been appropriate for a 150-foot shot. Did a refocus and discovered the beam was going around a limb about one-inch in diameter as the crosshairs were nearly dead center on that little limb that was only about 30 feet from the total station. Had the rodman raise the prism until the crosshairs were definitely above the limb and hitting no leaves. Boom! 149.77 feet! Exact agreement with the record dimension. Mighty close to being exactly 10 meters off.
Allright, Cow Thighs, I can understand the error, that, incidentally, would never have occurred if you had actually chained it, but......
I'm sorry, I just completely lost my train of thought.
I'm sure it was a very good point about the way things used to be, but maybe I just wanted to use the term "cow thighs."
Yeah, I'm pretty sure now it was the cow thighs thing. I liked that, even though I'm not really sure that cows even have thighs. I know they have calves, right? But I'm not talking about babies, you know, I'm talking about shapley legs, enticing buttocks, alluring breasts.....
O.K., cows don't have breasts, and buttocks are questionable. I know all this and yet, and yet, I can't stop talking about cows...
Help me, please....
Don
Time to put the cork back in the bottle, Don 😛
H.C. it's tempting to think it is a 10-meter cycle difference, but it doesn't come out quite close enough.
182.67 - 149.77 = 32.90 ft = 10.028 meters.
I'd expect that a cycle error wouldn't be an extra 2.8 cm = 0.09 ft off. Do you have another way to explain it?
> Did a refocus and discovered the beam was going around a limb about one-inch in diameter as the crosshairs were nearly dead center on that little limb that was only about 30 feet from the total station. Had the rodman raise the prism until the crosshairs were definitely above the limb and hitting no leaves. Boom! 149.77 feet! Exact agreement with the record dimension. Mighty close to being exactly 10 meters off.
A learning opportunity around every corner. Are you saying that the refraction of the beam added the 30' to the shot? I've made shots through branches and limbs close to the instrument and wondered whether the refraction around such objects could affect the shot, but I would have thought the delta would be small.
If the beam was measuring the distance to the limb, why wouldn't the shot just be 30'? How do you get a 10 meter lengthening of the shot?
Excellent questions so far....well, except for you-know-who
I do not know the specifics as to how the error occurred. Just reporting the facts.
If it is a 10 meter glitch, my guess is that the reading was taken before the rod was truly stabilized resulting in the 0.028 mentioned above.
If it is based on some weird bending of the time-space continuum by the intervening maple branch, it is merely a coincidence that the difference was so close to 10 meters.
Excellent questions so far....well, except for you-know-who
Could be the vertical angle, so that the slope distance is actually 10 meters off.
Excellent questions so far....well, except for you-know-who
Was the shot to a reflector with reflector-less mode switched on? I've made that mistake before.
The error is obvious. The squirrels in the gun use a Gunther chain. One of the little buggers got upset because maples have no acorns. This caused him to drop a pin...
Excellent questions so far....well, except for you-know-who
> I do not know the specifics as to how the error occurred. Just reporting the facts.
OK, so let's open the question up a bit:
We have heard that "heat shimmer" can affect edm (or angle) readings. Could a solid object close to the instrument cause errors due to refraction...I think the more proper term is diffraction, because it's around the edge of a solid object, as opposed to going through the transition of one media to another of different density.
AFAIK, waves of all frequencies...LW, SW radio, light, sound, etc. are all subject to the phenomena. Why wouldn't light (or IR) from a Total Station be included.
Anyone else ever seen this or (even better), quantified the possible effect?
Enquiring minds want to know.:-)
Excellent questions so far....well, except for you-know-who
If the reported distance was short of 10 meters, it could be a cycle slip in the slope distance and then reduced to horizontal. But the reported distance is more than 10 meters.
Movement of the branch, causing intermittent blocking of the light path could perhaps contribute to the problem.
Excellent questions so far....well, except for you-know-who
That is not a possibility. Try again.
Apparently it is an unsolvable mystery
Maybe it was all a dream sequence sort of like Bobby Ewing in the shower 30 years ago. (OK, perverts, stop that, I'm referring to the TV show Dallas)
Seriously, I thought some electronic wizardry guru would pop up with a sound, documentable reason.
Apparently it is an unsolvable mystery
"Seriously, I thought some electronic wizardry guru would pop up with a sound, documentable reason."
Excellent questions so far....well, except for you-know-who
The temperature and pressure conditions that cause refraction could put a few parts per million into the distance measurement. Diffraction would probably have even less effect. Neither accounts for 10 meters plus another 1 in 2000.
Apparently it is an unsolvable mystery
I do not know the answer but I believe what is happening is the edm sends out different pulses at different infrared frequencies (wavelengths) to resolve the different integers.1 pulse for the integer for 1,000 meters; 1 for the 100 meters; 1 for the 10 meters etc... In this instance it didn't resolve the 10 meter integer correctly likely due to interference of the intervening obstacles. I would guess it had more to due with the return signal strength being very low. Using some older edms and on very long shots to single prisms, I have experienced an exact 10 meter bust but have not experienced this in the last 10 to 15 years.
15 to 20 years ago with a topcon 302 gun I would see a 30' bust when shooting through wire fences.
