Topcom and Trimble lines not matching

[Lexion770]

I was told the other day that even on RTK if you create an a-b line on gatekeeper and copied it to a tractor with Trimble steering and a tractor with a Topcon steering system they would not steer on exactly the same line due to the different algorithm that the 2 systems use. Also if you took a a-b line off one screen and transferred it to the other screen they still would not steer on same line.
Is this correct. Has anybody run into problems having 2 different systems on farm and trying to move to permanent tramlines.
Thanks Tim

 

[Briantee]

Yes they are different. It depends whether or not they use UTM coordinate conversion, and which dataset if they do. UTM is a grid like representation so you are able to do math via the usual x,y plot. Trouble is, think of cutting up an orange into wedges. Now stretch the peel out and then put a grid over top of that. In order for north to be north, you have to rotate the grid so going straight up the y axis (to the point of the peel) is 0 degrees north or 180 degrees south as you move to the edge of the peel. This rotation is called convergence angle. There are a lot of different ways to arrive at that grid, and some systems just stay in Lat Lon even, so there will be discrepancies in what north is. Other systems don't even use convergence angle. And of course, no one says how they do anything.

I am unaware of software that will convert from system to system, but you could try manually calculating convergence angle (UTM Declination) and apply those corrections to your heading in the different systems.

In order to do that, here is the formula I use in AgOpenGPS (which matches Trimble and Outback)

γ = arctan [tan (λ - λ0) × sin φ]

where

γ is grid convergence,
λ0 is longitude of UTM zone's central meridian,
φ, λ are latitude, longitude of point in question

You can also determine the declination on this site - but what the OEM uses for calcs calculating this may or may not help you anyway - but now you know why they are different between mfrs.
https://geomag.nrcan.gc.ca/calc/mdcal-en.php

 

[Lexion770]

Thanks for that. A very comprehensive answer well explained.

 

[Pheasant Surprise]

In a nutshell, if tracking the “exact” AB (like to the inch) is important to you, then the only guarantee is to use the same guidance/manufacturer across the machines if at all possible!

There are other discrepancies that can become apparent - not only what @Briantee has mentioned but rather more boring stuff - like different mounting bias and different vehicle geometry of where the guidance places the reference position with respect to vehicle and implement for the AB line.

Straight AB lines, less of an issue, there’ll be some offset error (which you can correct) but following any curved AB lines will show up any differences and this is not accounting for any terrain compensation differences either for slopes and hills.

 

[James 6920]

Just red this thread, we run 3 JD RTK units 1, Topcon and now a Ag leader unit all on RTK correction from the same base station. Now we have had issues in the past with A-B lines not matching up but that was purely down to different base stations being used, now we’re all on the same base, does this mean if Topcon unit puts in A-B line tramline, and then those A-B line coordinates and heading are inputted in to Ag leader and John Deere panels, they won’t be tracking or positioned in exactly the right place?? As at the moment we seem to be ok? But we do have some discrepancies on some A-B lines. Also can’t seem to amend the coordinates on the Ag leader in command panel which could be an issue