If you were tasked with a trig level transfer of elevation from a benchmark at ground level, up to the 7th floor of a hotel, under construction (windows not installed) - would you want to shoot it both ways? Meaning shoot up to a prism on the 7th floor and then set up on the 7th floor and shoot back down to the benchmark? I'm wondering if refraction due to different temp gradients would be an issue in such a scenario, making it wise to get reciprocal measurements and average. Would it be an issue? Or am I reading too many geodesy books?
Working for an engineer who didn't want to more than double the time, or carry the gear up 7 flights of scaffolding. (Yes it's already done, from the ground only, but I wanted to shoot back down to compare)
It's about a 30m height difference - set up about 50m away from building - can see benchmark from 7th floor - stairwell and lift shaft not available for alternative solution. Everything shot on 2 faces. Building is pre-made panels so we are just checking/confirming what the elevation should be, that any creeping gain or loss over the 7 story's is acceptable.
So yes, it's mainly refraction I was worried about. Yes, No?
As long as it is not skimming any surface (building side, sidewalk, etc) I would not worry too much about refraction over that short distance. I am assuming the line of sight is going through open air.?ÿ
Of course, a reciprocal observation can never hurt, the more data the better! It can be useful as a good way to estimate the achieved accuracy.?ÿ
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It sure cannot hurt to check it from the building.?ÿ
What are the repercussions if the single shot is wrong?
Did you use a tilting prism?
Ken
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Do a bunch of checks, do it both directions, make sure what you are looking at for zenith angle is directly over the point. Make a sight that is thin to look at. You are going up +-70 feet so it's not a big deal for trig levels, the flatter you can make it the better. Take as much heat shimmer out of the equation as you can and do it before sun-up or a very cloudy day. The rule of thumb was always 490' and you get .01' of C&R (that being a bit over .005'), of course local conditions really dictate that, looking along a runway in the middle of a sunny summer day is different than to close hilltops before sunrise.?ÿ
Ask the Engineer if he wants the right answer or just an answer.
We all know he neither wants the right answer or just an answer: he wants $350 worth of answer and not a penny more.?ÿ
My experience is the floor to floor difference is more important than the aggregated difference from the ground to the 7th floor. For constructability, a floor to floor difference of 3mm (~ 1/8") is a very workable number. However if the 3mm accumulates each floor for 7 floors you have nearly an inch which is a difficult number to work with. You might see that accumulation if only shooting ground to 7th floor and can cause unnecessary alarm. Best practice is to carry elevations up as each floor is constructed.
And from my perspective, the vertical angle you use for trig is much more important that refraction. The farther back you go for a flatter angle the better your result will be.
And... using a steel tape for a direct vertical measurement is more accurate still. Measure either over the edge of the 7th floor or right up the elevator shaft (or stairwell.)
Edit: A couple of other considerations: 1) how high are you going? If you have a 30 story building, you'll have crush (in other words compression) to account for. 2) Settlement. Again it depends on the mass of the building. I've seen settlement on buildings of 2-1/2 inches (60+mm) when the floor to floor was less than 3mm. Made for a hard time for the door jam installers at the ground floor,
I'm not intending to be too hard on the Engineer ? . It was shot forward and reverse, yes to a tilting prism,?ÿ and actually, with 2 different guns (mine and his), and not every application of trig levelling requires reciprocal measurements. I was just wondering if this particular scenario would introduce refraction errors due to the ray travelling through different temperature gradients between ground and 30m up, which would cause reciprocal measurements to be recommended.
If you look at a typical diagram depicting temperature gradients -?ÿ there seems to be one every foot or so..there's so many of the pesky buggers, every one of them bending your ray in unpredictable ways! It's like equipotential surfaces - there's always another one coming..
The way I do it is setup on a random point, I shoot to a stakeout prism (0.100 or 0.440 m HT) rather than tripod to tripod. Use the same setup on the lower point and the upper point. The actual HT does not matter, it cancels out in the comps. You can setup at several different locations that have visibility to the upper and lower point.?ÿ
If using tripods, I would still take a shot to a prism pole or leveling rod at each level rather than depend on the HI/HT measurements to the instrument and prism setup on tripods
It was shot forward and reverse, yes to a tilting prism,?ÿ and actually, with 2 different guns (mine and his), and not every application of trig levelling requires reciprocal measurements.
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What were the differences?
We all know he neither wants the right answer or just an answer: he wants $350 worth of answer and not a penny more.?ÿ
right, you just tell the Geomatics Attendant at the Geomatics Filling Station how many dollars worth of Geomatics you would like today.
It was shot forward and reverse, yes to a tilting prism,?ÿ and actually, with 2 different guns (mine and his), and not every application of trig levelling requires reciprocal measurements.
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What were the differences?
4mm. My 3" gun, his 5" gun.
fixed height prism poles - no measure up to total station, just delta elevations backsight to foresight.
But if there was refraction it would have affected both ??ÿ
I think I'm obviously worrying about something that doesn't matter in this case.
It's hard to imagine the temperature would be any different from the ground floor to the 7th (especially when it's -15C [Canada])
A vertical angle of over 30 degrees is pushing the envelope with respect to trig leveling - at least that is what has been shared with me in the past.
A vertical angle of over 30 degrees is pushing the envelope with respect to trig leveling - at least that is what has been shared with me in the past.