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Hi everyone,
I´m doing some kind of experiment with an excel spreadsheet:
1-GPS ECEF adjusted coordinates converted to a local model based on height control points, using excel and only rotating and moving the adjusted ECEF points (no scale adjustement, no deformation).
2-GPS grid coordinates adjustment with TBC using the same height control points as 1
3-The results are almost the same in a 1.5km radius (+-0.5cm to 3cm), but at 2km or more, it looks like if TBC were fixing the adjustment to the control points introducing some deformation in the process.
The excel spreadsheet uses solver with the least square adj, method, so it is approximating one surface (gps) to another one (control points).
No geoid model was used, only ellipsoid heights in both methods.
The question is: What kind of adjustment is TBC doing? is it a combination of an inclined plane with a surface approximation?, only a surface approx?, any other?
I`m thinking that TBC treats each point independently from the others, although they belong to the same job and they are related to the same base.
Thanks
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Replies
As you said - no scale factor , no deformations in your excel calculations, but what about scale factor in your TBC local site ? Normally in wide areas- more than 2km, scale factor must be close, but not exactly 1.0000000000 ! Scale factor represents coordinate system deformation according to map projection and ellipsoid . Coordinates of control points are based on it. In wide areas we must take account at earth curvature, using direct ellipsoid transformations and predefined coordinate systems or local sites with their scale factors. If you force in TBC scale factor 1.000000000000 I'm sure all your results will be quite closer, but i wouldn't work with such kind of data frankly say... Using scale 1.0000000 you automatically refuse to take account at earth curvature witch is incorrect for areas more than 2km. Least square adjustment is perfect , but it's not doing the same job.
Regards Iliya
Iliya,
Of course you are right about the scale factor, but what I'm trying to do is to find why I have those differences, remember that I'm not using a map projection, only ECEF coordinates, my scale factor is fixed at 1 (data collector, TBC and excel), and my control points were fixed using an autolevel (1mm/Km), I'm checking height values only, not planar coordinates nor 3D coordinates.
What I've found on my lasts experiments is that TBC introduce an uneven deformation on the measurement area (2km radius), may be a scale adjustment but not a linear one, some points have more deformation than the others so here comes again:
How does TBC achieve those results?? I want to know the basic process lets say Least squares, surface aprox or whatever it is.
I'm aware that what I'm doing is a comparative analysis of 3D orthogonal coordinates (ECEF) vs. an aproximated surface considering it planar were it isn't (earth, gravity, etc.) this is an ellipsoid to geoid aproximation. With this in mind the differences are minimal... few cm. The point is how these few cm are distributed along the surface and why.
Thanks for your help.
I'm sure that someone at Trimble is reading this too, so come on guys tell the secret!!!.