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Question about GPS calibrations

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Jefferson73
(@jefferson73)
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Here's the gist of it... We're using Trimble R12i's with a TSC7. We have a multitude of calibrations city wide, but we just found out our survey needs to extend about 250m past our calibration. What kind of error would we have if we surveyed past our limits? Or, if we added a couple points to our calibration and extended it to fit the survey, would that change any of the data that we already surveyed from the original calibration? Also, is there a recommended distance between points for calibrations? 

 Thanks in advance, 

Jeff

 
Posted : November 10, 2022 11:57 am
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MI-Other-Left
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Are you more concerned with the horizontal, or the vertical?

 
Posted : November 10, 2022 12:24 pm
Jefferson73
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@michigan-left Definitely horizontal

 
Posted : November 10, 2022 12:25 pm

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Lurker
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If the residuals of your current calibration are relatively low you will not see much more error introduced to your point 250m beyond the "limits". The higher the residuals though the more the error will magnify outside your limits. If you held 1 point for vertical with a constant geoid separation your vertical error will not increase. However if you created an inclined plane the potential for error is greater, although if the inclined plane is a pretty good model, your vertical will be fine. 

 
Posted : November 10, 2022 12:34 pm
MightyMoe
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Honestly, it's 2022 now, there isn't a need to calibrate.

It's old tech and should be scrapped. Get a good projection use a Geoid and if you understand how you can make the projection work with any accurately laid out control network. Then you never need to worry about expanding. 

 

 
Posted : November 10, 2022 12:58 pm
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Rover83
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GIF

You're gonna kill us all!

 

Seriously though:

We have a multitude of calibrations city wide

Why not a LDP across the whole city? No need to localize again.

 
Posted : November 10, 2022 1:25 pm
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Jefferson73
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@rover83 Were an engineering company that uses university engineer students for the survey work. They are not surveyors. Most of the engineers here have a small grasp of surveying. I've surveyed for 19 years but the position they hired me for is coordination and ensuring things runs smoothly. These calibrations were already here when I arrived last year. We use calibrations because we don't just survey in the city. We can travel 100 km(62Mi) in any direction and have all these already built and ready to go for these students.

 
Posted : November 10, 2022 1:35 pm
Jefferson73
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@lurker Thanks!!

 
Posted : November 10, 2022 1:36 pm
Norm Miller
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Posted by: @jefferson73

if we added a couple points to our calibration and extended it to fit the survey, would that change any of the data that we already surveyed from the original calibration?

I've certainly seen that happen. It can get ugly fast. I helped a party chief who did that and couldn't understand why the previously surveyed points at the other end checked several tenths (oops - make that a decimeter) different. 

This post was modified 3 weeks ago 2 times by Norm Miller
 
Posted : November 10, 2022 3:09 pm

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OleManRiver
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I would like to say if you did an incline plane you can comp your error from the tilts in those directions and how far out the 250m goes. I would also point out doing calibrations over large areas even if your point residuals look good please pay attention to the scale it does. Calibrations were not really designed for larger areas and especially when you have vertical changes that are not flat from across the site. Calibrations were designed for sites.  Project sites. And even then on small sites with great vertical differences you sometimes have to have more than one calibration to make it work on the ground. A city wide for me sounds more like static control work and least squares adjustment than a site calibration or localization.  I have been asked many times to run around and build site calibration in different counties so all one has to do is use vrs and tie to the county monuments. I always say the same thing WHY and You have chose the wrong person.  I know what can happen. And as others have stated there is no need to do this. They the site calibration and localization were designed to aid in using gps on an assumed coordinates system.  Like assuming 10000 5000 100. NEEl.  I know some counties claim to have and cities that there control is on state plane etc.  and they often state that any work done you need to tie to one of there monuments before a plat is recorded. Tie doesn’t mean you have to hold that value. Just show the monuments and your bearing and distance to it. Most of this is derived from county GIS. I have a county here that is covered with control. Some is NAD83 HARN some is NAD83 2007 and some who knows what it is but they don’t fit themselves and they don’t fit the NGS monuments they supposedly held as NGS values vs there value on same point is way different. I think I figured out some of it which was a meters to usft international foot blunder. Some has been scaled. From where i have not spent that much time yet to figure it out. That same county also uses ngvd29 or a resemblance of it. Talking about a nightmare to work with. I usually stay NAD83 2011 and gps derived ortho heights via geod18 and then do a vertical shift to ngvd29 after I don’t hold there value as gospel. Because two points 500 feet apart don’t match by .2 tenths of a foot.  So I think my rtk is just as good lol. 

 
Posted : November 10, 2022 3:10 pm
Rover83
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Were an engineering company that uses university engineer students for the survey work. They are not surveyors. Most of the engineers here have a small grasp of surveying. I've surveyed for 19 years but the position they hired me for is coordination and ensuring things runs smoothly. These calibrations were already here when I arrived last year. We use calibrations because we don't just survey in the city. We can travel 100 km(62Mi) in any direction and have all these already built and ready to go for these students.

Sorry I was being more tongue-in-cheek than anything else.

It still blows my mind that we (including the areas I work in) have had a solution to avoid localizing AND place everything in a consistent ground coordinate system that is rigorously defined and relates all projects to each other, for the past 20+ years and barely anyone has adopted it.

I concur with the others that extending a localization after the fact can be really dicey. My biggest concern would be the vertical, especially if you are using an inclined plane.

There's nothing wrong with starting a dummy job, loading up the previous calibration file, writing down all the transformation parameters, then adding some points, computing again and comparing the new transformation with the old.

It might be worthwhile to at least give it a shot, and if it doesn't work, just go terrestrial rather than GNSS.

 

 
Posted : November 11, 2022 6:36 am
MightyMoe
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If you want to see the effect of the calibration take a look at the Global and Local coordinates. You should find a point where the calibration is pinned. At that point the Local and Global coordinates are identical, or should be. From that point outward the two coordinate pairs diverge, this is how Trimble relates the system that's being calibrated to with the geodetic system GPS surveys with. That is the error in the horizontal, for the vertical compare the ellipsoid height to the orthometric height. Then compare that number with the Geoid Height at that point using the latest Geoid Model, you will get a feel for errors in the vertical calibration. 

It's impossible for anyone to tell you how expanding will effect the accuracy of a calibration, it will never get to the elegant solution found by projection. The calibration introduces a system error (the original control network) where a projection being a math model in theory has no error inherent in the system. The error is introduced during the surveying process. So why introduce a second source of error that can be eliminated.

If the control you're calibrating to is accurate it's possible to design an LDP (Low Distortion Projection) that will incorporate that control into it. Thus, eliminating a need to continue to re-cablibrate (shudder) and it can be extended without fear of error sliding into the system. Frankly, a few hours designing that LDP will save many hours going forward. 

 

 
Posted : November 11, 2022 7:33 am
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