We have completed the instrumentation at a certain dam 3 times over the last 3 years (for the Corps of Engineers). Currently, the Corps has contracted with a construction company to install prisms on the dam for constant real-time monitoring. We have been retained by this construction company for QA/QC. The question I have pertains to their setup. They set a Leica robot inside of a "hut" with a glass window (see pic). Will the glass cause any interference with the measurements? I realize that any measurements by this robot (if comparing to previous measurements by same) will be relative and interference will be negligible. However, we will be checking these prisms setup outside of this hut (not behind any glass). Would you expect to see differences?
Leica in hut (permanent setup)
our setup: Trimble 0.5" S8 on pedestal (and one of our crew chiefs)
Just thinking out loud here...Like I said before, the effect of refraction would be cancelled out by comparing data collected by the Leica. Collecting data outside of the glass, with no refraction effect, would mean that I don't need to account for it? Or do I somehow need to figure out what effect refraction has on the Leica data, and incorporate that into our data?
Yes I understand the effect would be cancelled out for all data collected by the instrument inside the hut. My thought is that when we collect data from outside the hut, with no glass, to compare against the data collected from inside the hut that there will be differences due to refraction.
Wow...I don't have a clue. I presume there are a lot of things that might affect a distance. Possibly height above the ground....height of part of the line of sight above the ground, barometric pressure, temperature, weather in general. (sunshine? Clouds?) Maybe vertical differences....ie: would a steep vertical angle from the instrument location be different from the foresight location? Would it take a mean of the two directions?
Also, isn't glass a liquid? I understand it it constantly changing in width at any particular spot....like a wave.
But I am guessing all of my little speculations are relatively immaterial. I trust you would take into account barometric pressure at both ends of a shot, and the temperatures for your ppm calculations.
Maybe also in monitoring change, many of these factors just factor themselves out when you are measuring from the same permanent setup each time.
According to our dealer (and confirmed by field experience), shooting through ordinary glass at moderate angles does not have any appreciable effect on distances.
Unless your tolerances are sub-millimeter, I doubtthere will be any noticeable differences.
Unless of course they've turned on the reflectorless toggle inside the box........;-)
Temperature, pressure, etc all play a role in every distance we measure and are always taken into account. This situation is unique. Usually, when monitoring movement, you are provided coordinates, stations and offsets for comparison. You measure the same points from the same location as was measured in the last observation. This time though, the prisms are newly installed and we are trying to compare our locations to their locations. The differences are small, but more than expected.
Just an observation,in the first picture you can see the dam off in the distance beyond the trees. I am just guessing but with the Leica setting that close to the glass and shooting that far of a distance,it seems like the slightest refraction refraction would compound to me. Can the Leica see the pedestal? It might be worth it post up a prism there and see how the Leica hits it.
Yeah the pedestal we set up on is only 28 feet from the Leica
OK - here's a suggestion. It looks like there is a lot of overhang on the building - probably to keep rain from blowing onto the glass.
So how about this. Once you have everything set in place, mark the spots on the glass where the line of sights are and then have a 1" hole drilled through the glass. If it's only a couple of holes you would still be able to maintain environmental conditions without a lot of trouble and you would not have to worry about any refraction or other anomolies.
I would say even though you have diffrences, I would think the change in distance would be more critical.
If there is a constant 0.01' difference between 'inside' and outside total stations maybe thats o.k..
If its starts creeping one way or another...I would think you'd have a problem.
Joe
I for one do not like shooting thru glass of any sort.
I was at the old Geodimeter repair facility in Chicago a few years ago, and the tech there told me they had some kind of special glass in the windows. They had prisms mounted outside that they used for calibrations. Know idea what was special about the glass, but apparently they thought it made a difference.
Pretty interesting question that I don't have an answer to. I doubt even your dealer really knows. You probably need to find the higher up guru guys (ie. PhD) at Trimble to know for sure.
Set up a gun and a target. Shoot distance 10 times. Put pane of glass in front of gun. Shoot distance 10 times. Compare results.
And then turn the pane so you are shooting 45 degrees through it and repeat both distance and angle. You'll see whatever delta distance you had with-without increased 40%.
You'll also have a sideways offset in the sight path that will make a difference in angle, equivalent to moving your centering off some fraction of the glass thickness.
Of course most of the replies assume that the glass is optically flat.
Perhaps it is. But wind pressure, temperature changes affecting it's framing supports, etc. can bend and warp it which will change it's direction of refraction. I'd cut circular holes in the glass where the regular shots were to be taken.
I think Leica pretty much invented motorized monitoring so I'd look to their recommended procedures, if you can find them.
The total station on the pedestal operating in direct sunlight or whatever is not a 0.5" instrument in that situation; it is a joke.
Maybe I missed it above, BUT it appears that nobody asked the obvious [first] question...
HAVE BOTH instruments been to the NGS calibration base line RECENTLY?
🙂
LoyaL
Not to minimize the importance .....
of a calibrated instrument, but just how critical is it in this case. If you use one instrument from the same location to the same prisms, what you are looking for is change in distance / angle. The correct distance is less important than making sure nothing changes in the setup.
Now please don't get me wrong and say that I don't believe in having a calibrated instrument, but in this situation I think it's the least of your concerns. Where it would be important is if you had to change instruments at some time and you wanted to make sure you were comparing apples to apples.
Not to minimize the importance .....
If anything ever goes wrong with the instrument, or there are requirements to recalibrate it at some interval, then you'd better have started with it in good calibration to minimize the future jump in readings.
Not to minimize the importance .....
We checked our instruments on the baseline about a month ago. Good to go there. Like someone mentioned above, we are comparing apples and oranges in this situation.
Why don't they just make the glass operable? They could easily install an electric operating system to open and close the glass window prior to and after each observation set. We've had electric windows available for decades.
JBS
