How to Survey in RS2+ Base to match Trimble Base coordinates

Hello, I am new here, and basically new to all of this stuff, and trying my best to get a good grasp at this. I am a row crop farmer and we have used trimble for years for our autosteer on our 8 Tractors. We have Trimble FMX1000 monitors running off of a trimble base station for RTK correction. We are moving off of the RTK base station and going VRS…FMX1000 connected to a RV55 Serra wireless modem. We are currently running off out our local Case IH Ntrip subscriptions to receive our new “RTK Plus” correction. During the switch between our trimble local base station and to Case IH RTK Plus all of our AB lines shifted anywhere from 1 to 3 inches, which for the most part isn’t too big of a deal, but we like to be as precise as we can on our rows.
So now I have a Reach RS2+ coming and want to run off of my own base station. I have tried a friends RS2+ out and our AB lines were nearly 5 foot off. How would I go about and survey in my new Reach base so that my AB lines dont shift during the transfer to a new base and line up with how we were on our original Trimble base station?
Thank you!

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Hi @FarmerMP,

Welcome to our forum!

How did you set up your first base? Usually, the shift comes from the base position that was set in SINGLE or the coordinate systems mismatch.

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The first base was surveyed in correctly. And this is coming from our Trimble dealer. He is very picky so I know it was done right.

Original AB lines made on surveyed Trimble base. And we have close to 150 AB lines throughout all of our fields.

If I run on case IH VRS RTK plus we have a 1-3 in line shift on every AB line.

If I run off of my buddy’s Reach RS2+ I had a 5 foot shift. Did not verify on more than a couple AB lines. He simply just set the thing up in his backyard and that was it.

I am having a hard time understand how this works.

If I created all brand new ab lines on the reach rs2+ there wouldn’t be a problem…but if I went back to Trimble base with that new AB line I would suspect a 5 foot shift again. So how does one survey in this new reach that I am getting?

Just to flesh out Liudmila’s answer a bit more: basically, there are different coordinate systems everywhere around the world that have slightly different origin points and ellipsoid parameters depending on the use you want to make out of them. For instance, WGS84/ITRF14 are two global systems that satellite constellations use as they orbit the entire Earth. Then you have more continental/regional systems like NAD83 (in all its flavors) for North America that better conforms to the shape of the Earth on that continent compared to the global systems.

The main issue is that they are all geographic systems with the same angular units and very close values. The shifts are overall very small, but that could be what you’re experiencing right now. There are two things I would suggest you verify:

  • How the base position is set in the Emlid Flow app. Is it manually entered with whatever coordinates you have for your base position or does the unit accumulate an average over a time period everytime it boots. If it’s the latter, the base position will change at every bootup. Obviously you want to insure you do the former if you want repeatability.
  • What coordinate system/datum that base position had been recorded in. If for example the base position you have from your old system is in NAD83 and you enter them directly in your RS2+ receiver, this might be why you have the shift which might correspond to the difference between NAD83 and WGS84.
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Thank you for the replies. Yes the original AB lines and base were all done on NAD83. So what I am thinking of doing is when I get my pole securely mounted in the ground with a 5/8 bolt welded on top. I will drive my tractor right next to the pole and move the receiver Trimble AG25 from tractor to the pole. Then go into the trimble monitor and record the Lat and long point. That should get me where I need to be. So if your saying there will be a slight shift in my ab line since I’m on nad83 vs what the emlid is running on… Can I just tweak the lat and long until my ab line is dead on?

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You should be able to just transform the NAD83 coordinates to ITRF14/WGS84 and enter the result in your RS2+. If that was the issue, your shift should mostly disappear. There might be issues because of the observation epoch - I don’t know how long ago the base position had been surveyed - as there is a time component in certain reference frames that you’d need to take into account. You can try with this online tool: https://tagis.dep.wv.gov/convert/. I doubt the shift would be critical though and your problem would be solved, you could use the lines you have immediately if the shift is within your tolerances.

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