Emlid

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What kind of absolute position accuracy is typical?


#9

“Labor as in it takes at least 2-3 minutes (or more) to be confident in a point”

Is this with fix or float because the default collection time for float is more like this, the default on the Reach for fix is 40 seconds. When collecting points with a good fix and lots of satellites, I see the RMSE on the rover read out to be around n the 0.001m order of accuracy or 1cm. I would think then if you were able to establish a base coord position <5cm accuracy then your collected rover points should be <10cm? With a good fix, I’m wondering if even 10 or 20 seconds of collection would get you 10cm absolute position accuracy? No conversion of coordinates, talking only WGS84.


#10

So are you using two Topcon units (base and rover) or are you using a Topcon as a rover and a Reach as a base, or are you just using one Topcon that is using other means of correction (NTRIP, multiple CORS) or some other sort of correction network subscription? CORS can only be used for PPK right, not RTK and you say you are only doing RTK, no post processing?


(Michael Lambert) #11

Correct. Trying to get anywhere close to L1/L2 accuracies is what I am testing and giving you the real numbers on. The RMSE can say whatever it wants because that is what it has at that point in time. Try shooting the point, walking off about 10m and then staking that point out again. I think 60 seconds with a good fix will give you good confidence for less than 10cm. Because of the Topcon software I run I can be repeatable withing 7-10cm with a good float on the verge of a fix.


(Michael Lambert) #12

All field tests have been with a Topcon Hiper V base/rover kit vs the Emlid Reach RS+ base/rover kit. Our scenario is probably different than allot of you all because I (1) Do not have allot of trust in our CORS system after testing unless I post-process everything, (2) I need site acuracy and (3) The CORS system is not fast enough for our machine control. I think I will make this a disclaimer on my account as a little different of a user-base trying to push the Reach system or at least take it where it may not be intended.


(Christian Grüner) #13

Interesting scenario, I’ll try that soon!

What is your take on mechanical inaccuracies? I.e the levelling of rover-pole, threshold of a levelling bubble and so on?
I see a lot of images of people just hand-holding the rover-pole. Having tried that, and thus knowing how hard it is to keep that thing perfectly level for just 60 seconds, aren’t we then past 2-3 cm error quite quickly?


(Michael Lambert) #14

Yeah, that’s a big no-no and I highly encourage everyone to get get a rover pole with a tripod attachment for their base stations and a bipod for their rover. Don’t get me wrong, level tripods and a tribrach plus a mounting adapter are great, but for less cost you get an unmistakable, on-point, fixed height level in 30 seconds every time. Of course like anything else they need to be calibrated once every 6-months or so. I like the Seco yellow aluminum snap-lock poles. You can hunt around though and find better pricing.

https://www.tigersupplies.com/Products/Seco-Construction-Series-Thumb-Release-Tripod__SEC5218-40-FLY-.aspx

https://www.tigersupplies.com/Products/Seco-2m-Snap-Lock-Rover-Rod__SEC5125-20-YEL.aspx

https://www.tigersupplies.com/Products/Seco-Construction-Series-Thumb-Release-Bipod__SEC5217-40-FLY-.aspx


Tripods
(Michael Lambert) #15

Further on the testing. I setup each base on known points and have a quick-lock setup for my rovers so I can push-button move them from my vehicle mounts to my rover pole. I drive to the point, take the Reach off the car, lock it to my rover pole, shoot it and while leaving the pole there do the same with the Hiper. The first time I tried this I was using magnet for the Hiper and Reachview for the RS+ and I thought I had just wasted my money because my elevation deltas were huge, but what I quickly relaized is that the Reach is a theoretical ellipsoid where as my Topcon was on a localized datum. I had 6 feet of error from the two! I used the NOAA site the try to transform from the ellipsoid to my GEOID and that was nonsense. It was much closer, but still not the same as the level-loop.


(Christian Grüner) #16

That’s also what I use (tripod for base and pole+bipod for rover).

So for the sake of making a proper comparison, how much time do you spend with the pole at center of the GCP?
2 components as I see it, levelling and measuring, placing and leveling taking 15-30 seconds and then the time spent collecting data in a now static environment. Is it roughly comparable with your time spent ?


(Michael Lambert) #17

Sorry to hijack the thread…

Car Mount - https://www.tigersupplies.com/Products/Seco-Single-Mag-Mount-with-Quick-Release-Tip__SEC5114-051.aspx (They also make a tri-magnet for rough conditions.)

Rover pole - https://www.tigersupplies.com/Products/GPS-Quick-Release-Adapter-(Leica)__ADI765-11.aspx

Rover - https://www.tigersupplies.com/Products/AdirPro-GPS-Quick-Disconnect-Adapter__ADI765-02.aspx


New to Reach RS×
(Michael Lambert) #18

Leveling and placing not included, just the collection. I was trying for 2 minutes Emlid and my typical (don’t laugh) 3-second Topcon. I think I am not quite the normal user on this forum simply for the fact that I want to be relative and right on the ground inside the boundaries of my subject. Not the planet.


(Michael Lambert) #19

All that being said, I totally support Emlid. I think it is an amazing product for the cost-point and if I can get it to do what i want it to do with our Magnet Field software with minor user training I will order 20 sets tomorrow.


(Christian Grüner) #20

So in relative terms, you would expect 1.5 mm XYZ combined RMS from the topcon from a topcon multiband system with 3 seconds, if I understand you correctly ?


(Michael Lambert) #21

Correct, this is with a control system localized from control per the engineering plans which is shot in by a robotic total station and level-looped for elevations. Fairly typical construction process, at least in our company. Where my tolerance is and may be perceived differently is that anything more than 0.05’ deviation in the localization file is removed and my stakeouts can then be 0.05’, NEVER more than a tenth. If a control point checks in that bad then it was either mistakenly used or it has been disturbed. and is taken out of the localization after some checks. Usually 0.03-0.07’ is pretty typical for our primary control.


(Christian Grüner) #22

So 0.8-2.54 mm for us Europeans :smiley:

For the same 3 second obs time, what would you, with your experience of the system, expect of the reach ?
And for a 30 second obs time?


(Michael Lambert) #23

Good fix with the Reachview software 7cm and Magnet Software 5cm. RTK, no post-processing. The only place I really have trouble with the Reach, because of the single channel, is in our more hilly areas with medium vegetation.


(Christian Grüner) #24

so to standardise the test, would this flow be ok for a comparison:

  • initiate base (average single fine, as the test is about relative accuracy)
  • initate rover (note time when ready), kinematic RTK mode
  • start new survey
  • obtain fix (note time, note how long it took to fix AR > 3
  • walk to first GCP
  • place Rover rod and bipod on GCP
  • level setup
  • survey for 3 seconds
  • stop and save collection
  • survey for 40 seconds (Emlid recommended)
  • stop and save collection
  • survey for 3 mins
  • stop and save collection
  • place rover on next step and repeat the 3 individual measurements.

#25

Would it be best though to simulate a single collection, to move the rover then come pack to the GCP then do another collection? Isn’t the RTK running continuously so if you leveled the bipod (several seconds) made a 3 second collection, then after a few more seconds made a 40 second collection, wouldn’t the 40 second collection theoretically be more corrected than if you you made a 40 second collection as soon as you placed and leveled on the GCP since the rover would have been sitting on the GCP for longer if you are taking a succession of multiple collections at the same spot?

I was noticing with a good fix, that the RMSE numbers were the same (going to 3 decimal places) whether I collected 10 seconds or 1 minute. They didn’t go significantly up or down (I was working in pretty ideal conditions) so my thought was the 10 second collection (at least for my purpose not needing <3cm accuracy) was adequate and collecting any longer is following the law of diminishing returns on the accuracy over the extra time to survey multiple points.


(Michael Lambert) #26

Are you testing just the emlid or against another survey system? I think your outline looks pretty good. @jazee had a good point with walking away from it, but you’re test seems designed to prove that a longer collection time is more repeatable. That being said once you have collected your three points walk at least 10 meters away, sit for a second and then go back to the point and stake out each of the three points that you just collected. The longest point should be the most accurate, but if there is not a differentiation that exceeds your tolerance then you know on average what is safe to shoot. I would do this several times on different points.


(Michael Lambert) #27

I think for the accuracies that you are needing that with a good fix 60 seconds should be plenty.


(Michael Lambert) #28

@timmyd, have you shared the link to your video yet? Do you know the timestamp for the basic EZSurv run through?
If it is posted then I would encourage everyone to go look at the workflow for EZSurv. If your use-case is to make money then the bit that you spend on this program is a better choice than RTKLIB. Especially if you have more people than yourself that are going to be dealing with this data.