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averaging observations after calibrating site  

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cole
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 cole
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when setting up a job with GPS (off the VRS) i typically observe all control points for 3 minutes, then take two that span the scope of the site (preferably in a line such as a prop line, section line...)and use those to calibrate horizontal. then ill use a city benchmark to establish vertical control.  however, sometimes i find when i come back the next day to start staking improvements, i will have variances of up to a tenth, mostly vertical, but sometimes 5 hun or so horizontal as well. to improve accuracy, before staking improvements i will re observe all control with 3 min obs and avg the shots.  today, i noticed the coords did not change at all though for my point that i had previously used as horizontal calibration.  

my question is, once you calibrate the site, are those points that are used in the calibration not affected by averaging observations? if not, does this make avg control points a bad idea, as those used in the calibration may deviate more from those that are averaged multiple times through different 3 min obs?  thus, creating more discrepency between control used in calibration, and control not used in the site cal?

only been at it for 2 years now and im thrown to the wolves with no mentor, so i gotta figure stuff out solo.

 

thanks , cole

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Rover83
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Not sure what software you are using, but most major brand software (I know Trimble Access for sure) does not recalibrate on the fly when control points are re-observed. The idea is that all points are observed to desired precision, and then as many as possible are used in the calibration to minimize residuals. (It is highly recommended to use more than just two points. Four points spaced around the exterior of the site is the rule-of-thumb minimum.)

Once the calibration is computed, it is just a set of transformation parameters to go directly from the raw GNSS measurements to local values, so it is not "looking at" the control points any more. So it will not change until you tell it to re-calibrate.

Re-calibrating can be VERY dangerous if you are in the middle of performing stakeout. Read up on how your software handles it before making that decision. Changing a calibration midway through a job is dicey at best.

 

Also, it all depends on what you are doing, but 0.05' horizontal and 0.10' vertical is pretty great for RTK, even network RTK. Well within tolerances, at least in the areas that I have worked...

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cole
 cole
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@rover83

i guess what im thinking is to use 2 points and create that baseline to which everything else is relative. if i do 4 point calibrations, then i have deltas between calc points and all of my observed points. (i am open to new techniques, please explain why more points are better).  having a tenth here or there vertically is not compliant by my standards, especially when providing benchmarks for contractors who are using that to build a finished floor. even flowline of gutter or gravity flow pipe may be staked incorrect if your setup is a tenth off vertically. probably wont wreck a site or anything, but def gets my OCD going. again, im only 2 years in, i may be wrong about everything i just wrote. 

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Ricky1947:)
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@cole

I’m thinking if a tenth of a foot vertically isn’t good enough for you or the project, you shouldn’t be using RTK to perform the survey. RTK is great and cool, but it’s just a tool. If the tool can’t perform the job correctly and safely, use a tool that will. Site calibrations, in simple terms, best fits your measured data to the record values of the same reference points. Maybe think of it this way, using more points in the calibration increases your confidence level of the entire project being acceptable for accuracy. If I hinge a job only on one point, my confidence level is quite low that the rest of the project is accurate. What if that one point was the only point that was disturbed by the night shift dozer operator? You need more points to calibrate to to increase your confidence that your work is accurate. It also allows you to see points that are not acceptable for accuracy, you have the ability to dismiss them. Calibration scales, rotates and translates your measured job data to a predefined coordinate system. When you calibrate using two points, the first point in the calibration translates your job data to that position. When you add a second point to the calibration, a rotation and scale is applied to the job data. When your measured inverse data does not match the record inverse data ( and it most likely will not) this is when a scale factor is applied, ideally you would want this scale factor to be as close to zero as possible so when you lay something out at 200 feet, it will measure 200 feet. 

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David Livingstone
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There was a recent post about the accuracy of VRS. It seems the general consensus is the vertical accuracy is not as good as a base rover system. We just bought a VRS system and I agree it’s not as accurate as base rover. It also depends on the quality of the VRS systems and how close some bases are. 

 

I also agree  I wouldn’t change a calibration once done. I’d also say a calibration doesn’t increase the accuracy of a VRS.  I rarely calibrate any more unless forced to by control not on state plane. 

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Loyal
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Posted by: @david-livingstone

I rarely calibrate any more unless forced to by control not on state plane. 

Just curious, but what does "state plane" have to do with anything.

Loyal

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Bill93
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That should probably read "not referenced to the same datum as the VRS and on a well-defined projection that the data collector can handle."

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David Livingstone
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I was bringing up state plane because I wasn't sure the OP thought maybe doing a calibration would make the VRS better and more accurate instead of working in state plane coordinates.  I'm not sure if that was what he was thinking or not.

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