Scaling Distance Error (Nikon Total Station)

[Trestkon]

Hi everyone! I'm hoping someone can shed some light on an issue I'm having, because I've run out of ideas.

I'm a surveyor for a construction company and I've just moved to a new city and I'm attempting to get my current job control points all squared away. Here's my saga:

-First day here I checked the legal survey with my the Nikon 632 that they had on site. I came up with a shortfall of 8mm between two of their points (over 52m) and slightly more between two others that were 61m apart.

-I got the legal guys to come back and re-confirm their survey, which they did. Their Leica had been calibrated a week prior to coming to site so everything on their end should be perfect.

-I assumed the EDM on my machine must be out, so I sent it away for calibration and got a loaner Nikon Nivo.

-Same results.

-Got another loaner, same results.

-I moved the station inside, and on a flat floor, taped out about 45m. Station reported a distance that was 7mm short. I then tested at half that distance and the error was 4mm.

-I've tried these tests using 2 different backsight prisms, a brand new mini-prism, AND reflectorless.

-Temperature and pressure correction is on, prism constants are correct.

Both the rental company (Cansel) and a professional surveyor friend of mine I've talked to on the phone have no clue what the problem could be.

My only thought is that the stations I'm using are coming from 1 province over, which is about 500m about sea level higher than where I'm located. I don't imagine that would make much if any difference, but it's my only thought, at this point.

 

[IRstuff]

Are you sure you have the correct correction factor for the reflectors. Most such instruments have specific range offsets for each reflector to account for the depth of the retroreflector.

TTFN
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[Trestkon]

Your referring to the prism constant, correct?

If so, then yes. The prisms I'm using can either bet set to 0mm or 30mm, I've tried both. And the fact that the error is not constant (it grows with a longer distance) leads me to believe it's not prism related. I also tried prismless (with the same result), to further rule that out.

 

[IRstuff]

It looks not impossible that some atmospheric correction could be unaccounted for:

http://emtoolbox.nist.gov/Wavelength/Documentation.asp,

but unlikely. Your few data points in your OP nearly fall on a straight line for error vs. distance, so there does appear to be a scaling error. You might want to check a fixed distance over time, recording temp, press, and RH, and see if there is a correlation to the error. One other possibility is to measure the distance concurrently with the "legal" guys to see if you can spot some sort of difference between their approach and yours. Is your area known to have anything usual in the air, like excess CO2 or whatever? Achieving parity down to 100 ppm is not necessarily trivial. What is the generally acceptable error for your control points?

TTFN
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[Trestkon]

Thanks for the feedback, IRstuff!

I accidentally found another guy in our district that also has surveying experience, and solid control points on his job site. I'll be taking our station out there and shooting his points on Monday so we can directly compare our machines and settings.

I actually doubt that any air density issues could really be the cause. I ran through the calculations on Friday and at best it would be a mm or two. For our control points I'd definitely not expect more than 3mm. I believe the error on the nikon is 2mm + 15ppm in precise mode or something similar. In the previous city/job I was at we never had a problem agreeing third party surveyors within 1mm. Coincidentally they used a Leica, just like the legal surveyors in this case.