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I am lead to believe Topcon GPS users may only calibrate for elevation on three, or is it four, control points to establish a localised level datum and that the result is an inclined plane of best fit between the control points. I am also lead to believe the option to maintain a "constant adjustment" is not available as it is with Trimble stuff. What happens if one or two of the points is in error vertically or if these "one or two" points were not officially issued as "control"? Answer..."it" hits the fan.

Surely Topcon provides the option to create an horizontal datum plane by adopting only one point for level and other points would be used as checks only. I sure hope it does if I ever have to use Topcon.

Can anyone enlighten me while I do a bit of a clean up!

This Content Originally Published by a land surveyor to Land Surveyors United Network

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I can't comment about a "constant adjustment". It sounds cool but might require a bit of hocus pocus.

Any localization (gpe calibration) goes this way.

1 control point allows one to translate the project

2 control points allows one to translate, orient and scale

3 control points does #2 and allows a tilt trend and supplies redundant horizontal data

4 control points does #3 and supplies redundant data all around

more control points add to redundance and eases finding poorly fitting control.


I do agree with you JAC
Use one control point as a vertical BM. Then go and shoot each of the others and compare to existing control. See if you have a tilt or not. Then adjust from there. If some are high and some are low and they do not form a plane then you will have to make an educated decision from there what to do.

As a basic rule of plane geometry - you should always hit at least 4 points when calibrating.  Otherwise the routine  being used will or could show a perfect fit of the three points to the vertical plane.  By using four or more points you should get a residual error report for each point being a couple hundreds off the "best fit" vertical plane.  

Having not used Topcon GPS ( I've only used Trimble or Ashtech/Sokkia gear) I can only speculate as to the actual process.  But, I would always hit 4 or more points depending on the size and shape of the area I was working in.

Hope this helps

Some interesting replies, thank you all. I've got the basic rules down pat, after 50 odd years it's just difficult to come to grips with the notion of level datum plane not being horizontal over a small area i.,e., the RL of the datum varies depending on where you are. In the past I have established a site level based on a particular bench mark and checked it against a number of other marks, deciding whether I'm good to go after that. Kevin D. Allen in your Trimble equipment do you calibrate using a "constant adjustment" or an "inclined plane"?

I always used "constant" - I could never get over the idea of an "inclined" plane being level

The "mind bending" part is that the inclined plane IS level; IS normal to the direction of a  plumb bob.

The simple geometric position given by GPS doesn't know which way the blumb bob hangs; doesn't know which way is down. It is a global system and all directions are down at some point.


Yup - I should clarify - back in the "day" when RTK was new - I was one of the first in Indiana to use the Sokkia/Ashtech system dual-freq system that had the old SDR-33 data collector running the total station, digital level AND the RTK-GPS.  Back then the data collectors did not have the GEOID models loaded.   You had to do rapid-static ( 15 minute occupations) on your control points, either before or after you did you RTK work, post-process those and adjust to the local BM, then do a transform function in the data collector to pull the RTK data to the adjusted post-processed control, check the 4 calibration points for residual error and then download your adjusted RTK data.... Oh these  youngin's today have no idea what hoops we early GPS users jumped through

Ha Mr Allen, and they don't appreciate how hard it was sometimes to measure just a hundred metres through scrub and bush over hill and dale either.

Mr Cavell your average dozer driver would not understand the concept of level and horizontal. In a perfect world where the BM's are all in perfect agreement re; plum lines and "level" etc I accept the solution would be level or horizontal for the laymen. My initial point was that one of the BM's was in error and this was never picked up by said dozer driver hence we have some rectification earthworks to tidy up. Further, if the solution is by all accounts level then why does Trimble call it an inclined plane?

Aargh! thank goodness retirement looms.


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