Reach RS2 relative accuracy problem

Kia orana everyone

I am a civil engineer (no surveyor, so excuse my ignorance :wink: ) in the south pacific and purchased two reach RS2 recently to acquire the location of assets on very remote islands.

My problem is that I am unable to set-out a previously measured point correctly (less than 200mm) the closest I get is about 1m.

Setup:
Both RS2 were started up, connected to Emlid flow app and updated to the latest firmware.
Base setup: Positioning mode : Static, 1Hz Update; correction input: off; correction output LoRa
Rover setup: Positioning mode : Kinematic, 5Hz Update; correction input:LoRa correction output: off
Status on both is over 30 satellites and the rover is receiving corrections.

Methodology:
First I created a project, set the coordinate system to WGS 84 / UTM Zone 4S (Cook Islands) and the vertical datum to EGM2008 height.
Next I used the Rover in single mode and recorded a temporary survey mark (nail in concrete) within my project.
Next, I setup the Base over the temp survey mark, measure the height and then in base settings change the coordinate entry method to ‘Manual’ and load the previously recorded point.
Then I setup the Rover again and wait for the 'beep" to ensure everything is connected and ready.
With that in place I take a control point and save it in the project.

After that I turn everything off and on again, loading the same location point for the base again.

When I then try to set out my saved control point, I am always between 1-3m away from the exact location. The elevation also fluctuates widely and sometimes tells my I am 10m below the original point.

I would really appreciate any help to get this right.

Thanks
Felix

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Hi Felix.

It sounds like you are not precisely establishing you base point. Each time you set up on that point and average (single), you would get different values. If you are not using a benchmark with reliable coordinate accuracy, you first want to establish that point using RTK, PPK or PPP methodology. And then it would be suitable for using as your base and should get reasonably repeatable results with your rover.

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Thanks Dave for your reply.

My problem is that I will be using the RS2 on Islands where there is no survey mark whatsoever.
For this reason I had first established a survey point and surveyed it with the Rover without the base station (I know this is widely inaccurate in a global sense).
I then set up the base over the survey point and manually loaded the coordinates from the single RS2 as the coordinates.
I thought this would give me sufficient “relative accuracy” in relation to this point.
Once the base was established I surveyed a test point, turned everything off and on again (loading the same survey point into the base and tried to find the test point again.
However, I am still out by at least a meter…

Is there another way how I can establish a base mark?

The problem originates in the collection of the point you use for the base. Since a SINGLE solution is only accurate to within a few meters, it will introduce that inaccuracy into the corrections the base sends the rover. I understand you are really just looking for relative accuracy in your project. But because the base thinks it is in the precise position that is reflected in the coordinates you set, it is sending out corrections based on it being AT those coordinates when actually it is only somewhere within a few meters of that position.

If you have the time, you can use a Precise Point Positioning (PPP) workflow for anywhere on earth, to get a more precise point coordinate for your base.

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Rubbish in, Rubbish out :joy:

Thank you so much Dave.

I will work through the PPP process, that should be excatly what I need.

You made my day :wink:

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Hi Felix,

Welcome to our community!

PPP is indeed a great solution if you don’t have known points nearby, and it helps obtain absolute accuracy. But I wanted to share a bit more details on how relative accuracy works.

The rover always calculates the position with centimeter-level precision relative to the base. So, if the base is placed right at the same spot, and you use the same coordinates to set it up, you should obtain the same coordinates with the rover. These coordinates may be not accurate to the global ones, though.

Usually, the shift in the rover’s coordinates appears if you place the base on the same point but with different coordinates. For instance, if the coordinates are averaged with a few-meter level accurate Single solution each time on startup. So, I’d recommend double-checking that you use the same settings at each step. If so, please share the CSV file from Emlid Flow with the same control point collected before and after rebooting the base. I’ll check what can be wrong.

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I did not know that, thanks!

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